Move tests for frameworks-base-testutils lib into their own target

The test modules that include these util classes are running the util tests as well, because they're in the same java library. This change separates the util classes from their tests, with a new separate test mapping.

Fix: 292212651
Test: atest frameworks-base-testutils-tests
Change-Id: Ic8309a303307457e0e642370bc2a0a0335e4e8f6