Make library compatible with Android version < S and implement Github action for running tests on Android emulator

this-kramer commented 3 years ago

Reason for this PR:

We use BigInteger:intValueExact() which is [not supported by Android Version < S](https://developer.android.com/reference/java/math/BigInteger#intValueExact())
To avoid future compatibility issues, we would like to include Android testing into the testing workflows

Changes in this PR:

Use BigInteger:intValue() plus some additional checks to emulate the behavior of intValueExact()
Add Github actions job to run Math tests on Android emulator. This is implemented via an Android subproject that has the math root source set configured as its own source set and is therefore able to run the Math tests.

rheitjoh commented 3 years ago

Do we maybe want to add a range check for the BigInteger to preserve the previous behavior of throwing an exception for out of bounds values?

this-kramer commented 3 years ago

Do we maybe want to add a range check for the BigInteger to preserve the previous behavior of throwing an exception for out of bounds values?

Good point! Can you implement that or should I do it?

cryptimeleon / math