Specify SDK versions in makefile

Also makes the app runnable on non-Q devices.
(SDK version 'current' requires a pre-release build.)

Note min SDK is currently set to 24 as:
  - 19~20 fails with dex-limit error
  - 19~23 warns when desugaring media2-exoplayer

Test: build & run
Bug: 123716038
Change-Id: I7d0edda2160d0611b19fe8da1df45fd3b4153a3a
1 file changed