Fix Flaky MediaPlayerDataTest test.

MediaPlayerData relied on a static call to System.currentTimeMillis.
This somtimes caused the same value to be returned for subsequent
method calls in tests, causing flakiness. Pass in SystemUI's own
SystemClock instead.

Fixes: 192244434
Test: atest SystemUITests
Change-Id: I20868b5dfa50a2ece4c3261800076f1eb20f1bc7