BouncyBall: Remove some dependencies

To further simplify this app (which helps when building against
older SDKs), we remove our androidx dependency, as well as our
AppTheme.

Bug: 441133956
Test: App builds and launches as before across same devices
Flag: EXEMPT TEST_ONLY
Change-Id: Ib950c2d03a56c0f4e4b4f4f213c421117528d651