CombinedMessageQueue add peekWhenForTest and popForTest methods

The intention here is that tests should be able to call these methods
instead of reaching into MessageQueue internals via reflection.

Test: atest MessageQueueTest
Test: atest TestLooperManagerTest
Flag: android.os.message_queue_testability
Bug: 379472827
Change-Id: I5ff9dcac41f76aeabbce51f7d6db2ed5582c5664