MediaPlayer2: mark MediaPlayer-style DRM APIs TestApi

Keep MediaPlayer-style DRM APIs as `@TestApi`s to help test
regressions between MediaPlayer-style and MediaPlayer2-style
DRM APIs.

Bug: 123425309
Test: compiles
Change-Id: I880129eff1b12a1476be240f1566bb4eedc94a63
2 files changed