Cleaned up constructor reflection code.

There was a lot of duplicate code and tests between BurstAndroid and BurstJUnit4 for reflecting on constructors, refactored all that into BurstableConstructor and BurstableConstructorTest.
Yep, this can be removed. I did the field support first, and then edited this commit out of that. This method does more stuff in that commit, but I'll delete it here.