Folder-to-SFZ Convertor, made by the excellent Versilian Studios, is a free application which allows you to very easily make your own SFZ instruments from your own samples. Given that the SFZ format seems to be the one favoured by Android developers, it is an application that Android music makers should definitely be aware of.
It might seem a bit daunting at first, but once you get the idea it is actually extremely easy to use. Just take the time to watch the really helpful video guides on the Versilian Studios website and you'll be up and running. It's really just down to naming your samples correctly. There is also a downloadable user manual on their website.
For such an apparently simple application it actually allows you great control. You can set the attack and release for samples, include different velocity layers and even round-robin samples (multiple samples of the same note, with their minor fluctuations, to be played in order or randomly when that note is triggered, giving the instrument a more realistic feel).
It's the easiest way I've found to create a SFZ instrument with all of that functionality.
Folder-to-SFZ Convertor is platform agnostic but requires Java to run. I've tried running it through some Java emulators for Android without any success - if anyone manages it please leave a comment - so you'll need to use it on a desktop or laptop.
One really important thing to note though: it was pointed out on a forum by internet user peastman that the SFZ file produced by Folder-to-SFZ Convertor has an error which prevents it working properly on Android (and Mac apparently). It is really easy to fix though. All you need to do is open the SFZ file as a text file and delete a backslash (\) symbol before saving it again without it. Here's an example. At the top of the SFZ text file, well one line down, it will say
when it should be
(…obviously ‘Example Samples’ here is used for the purpose of this example only - yours will have the name of the folder that contains your samples instead).
The last time I used Folder-to-SFZ Convertor, this error in the resulting SFZ file was still there so please be aware of it - it's so easy to fix though so it's really not a problem.
Folder-to-SFZ Convertor can be found on the Versilian Studios website here