Add peekWhen() and pop() methods to TestLooperManager.

See: go/testable-messagequeue-bluedoc
Also promoted TestLooperManagerTest to CTS, and deleted
the unit test that's now redundant. See: ag/30617344

Bug: 379472827
Flag: android.os.message_queue_testability
Change-Id: I2d2b057dcc77fc75b27d937bbbf5de0702e2e300