Allow for replaceable clock within Looper

Handler, Looper, and MessageQueue are all difficult to test since their
internal implementations implicitly rely on the system clock. By making
their clock replaceable in tests, we can make their behavior more
deterministic, allowing for less flaky tests overall.

Also, update TestLooper to take advantage of the replaceable clock.

Bug: 184781936
Test: atest LooperTest MessageQueueTest HandlerTest
Change-Id: I215d2eab99888a162175c37157d7be2be911b96d