Remove unnecessarily @System/TestApi annotations

We can't expose APIs if the enclosing class is hidden, so these
annotations are redundant. We need to remove them so that we can enable the
check.

Exempt-From-Owner-Approval:Cherry-pick from goog/master
Bug: 159121253
Test: treehugger (i.e. this shouldn't trigger "API has changed" error.)
Merged-in: Ie1841a670bdf6c6f4b25a1fc5deed8ec2d18cda2
Change-Id: Ie1841a670bdf6c6f4b25a1fc5deed8ec2d18cda2