return a reason for RxFingerprint.IsAvailable

[ndori]

when a fingerprint is unavailable there can be several reasons, the hardware is not supported, no enrolled fingerprint.....

I want to show a reason for the user because he might be able to do something about it, (e.g. enroll a fingerprint, buy a new device...)

so instead of checking every reason one by one, just send the reason so developers can act accordingly

[Mauin]

Starting with Android P and the deprecation of FingerprintManager there won't be any way (that I currently know of) to determine the concrete reason. The reason will then be propagated as an error or help callback directly as far as I can tell. That would make this more in line with #93