Fix timing issue for flaky tests

Inject SystemClock so that we can mock it in tests and ensure the clock
will always advance. Also switched to using elapsedRealtime, which is
guaranteed to be monotonic

Fixes: 182813365
Test: atest MediaDataManagerTest
Change-Id: I133fb48e8242dc7f8e75bfa5c585c8b6142963aa
4 files changed