Split NotificationMediaManager out of StatusBar.

This class keeps track of the current media notification, and performs
various media notification related tasks. This is part of an ongoing
refactor to modularise StatusBar.

Bug: 63874929
Bug: 62602530
Test: runtest systemui
Test: Compile and run
Change-Id: I2f6c0c66f824e3ae24690794a5e8421c898ec355