This is a nice article / post from one of the oldest ( or the oldest ) websites still going strong regarding mobile music making and how he sees Android and its musical possibilities. 
One interesting thing is that he does not compare it with iOS but talks about Android on its own terms.
To read it go here:
http://www.palmsounds.net/2014/07/where-we-are-with-android-and-where-next.html

and extra props for recommending Musical Android!!!

If not using Audio Evolution Mobile with a USB Soundcard it is now a much improved latency correction built in as shown in this video.

Play Store link:
Audio Evolution Mobile

Almost all or all of Mr Nightradio's SunVox compositions only contains the internal synthesizer engines and no samples and it sounds that this is the case here as well.

There is sooooo much sample material to take from this video clips that are mostly from the late eighties. So if you need an endless supply of vocal phrases...

There is only two horror movies the last years or so that to me was interesting and this is one of them. 
Maybe not really so much of a horror movie as a metaphysical meditation presented in such a way that it can lead to some serious nightmares.

This album was made fully in SunVox on a Android tablet and just the 
final mastering made with Izotope's Ozone. 
Do like it a lot and did a interview with Jimsey that you can read here:
http://www.musicalandroid.com/interviews/jimsey-interview-with-a-great-sunvox-artist

The developer of Moon Synth and Psycho Flute ( both recommendable ) was recommending this and it was a good thing as even it is limited soundwise it sounds very good.
Actually it just contains three sounds but I am already in love with one of the sounds 
( an olden time electronic choir sound ) and see me using this as an instrument just for 
that sound.
It functions as most of the other Theremin apps and has the possibilities to adjust Octave, Scale, Key, Size and choose one of the three sounds. It is possible that many would not really like any of the sounds but for me like said am in love with one of them and being multi touch and with the possibility to directly affect velocity it can become a good thing to add to your arsenal of instruments.
Plus it is for free and no publicity!
EtherSurface

Ha ha this must be the first time there is a posting with a real crap app!
Have used this in some music as I like the cheesy but beautiful sound and want to start to mention some of the virtual instruments as well.
This is basically a app that have sampled some electronic choir and packed it in with a keyboard. Full of publicity to! 
That said it is a specific sound that for me is useful and will use it more than once.

Play Store link:
Best Piano Choir

A small update and one thing that can be useful if you work with a lot of samples is that it now has the possibility to tag them making it easier to find the sound you are looking for.
As alway mentioning this app it costs very little and have huge potential. Specially for you that want to have a workstation that takes different time signatures and ease of creating 
poly rhythms to a level that is not seen in any of the other bigger apps.
The update:
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Improve scale modulation
Filter samples by tag
Fix guides that do not divide evenly into pattern
Fix step selection after step toggled
Fix display on high-resolution devices
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Play Store link:
Syntheogen

Wrote about this coming some time ago and now it is here since two weeks.
It seemed exciting first but can not see how I would use it for making music as the bpm is fixed. It does have its uses and it is fun to play around with though.
The first thing to do is recording samples directly into your device. The sample recorded is some seconds long and it seems that it decides a little bit randomly the playback speed of the sample at least on my device so it may be a bug? 
( If it is a bug it adds to the surprise and randomness in a good way in my opinion. ) 
After this you have to decide a shape for the sample recorded and that is what decides the basic rhythm. Then you can affect each side of the shapes with reverse. down or none. 
You also have three sizes for the forms which will affect the playback. 
Volume and pan gets controlled where depending on where the shape 
gets placed on the screen.
One other thing is that you can not save, 
so if you work out something it would have to be recorded directly.  
Feel in the end that it could be good to have on your device for the one in a hundred occasions where you would need something chaotic or sit and play with it and record a lot and cut out the best parts to use later preferably in a app that can timestretch samples.

For free and without publicity.
There is also a Webapp version to try so for lazy people that do not want to install first can try it out...

Play Store link:
Geometric Music
Webapp link:
http://geometricmusic.be/webapp/

So there is some apps that can be called workstations with synthesizers, sample capabilities, multitrack, sequencer and so on but there is only one that is for free and that is VuKNOB.
Finally will take some time with VuKNOB as it is now being actively developed with three nice upgrades the last half year or so. 
The last upgrade contains:
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* New JAM interface, multitouch support
* Automatic chords
* Basic arpeggiator
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It will be fun to check out this the days to come.
Play Store link:
VuKNOB