Make PIP notification react to metadata updates

Currently, the PIP notification reacts only to changes in media
session playback state. This makes it impossible to make visible changes
in the media sesison metadata like the title.

This CL adds a callback to react to metadata changes.

Bug: 165920050
Bug: 153435517
Test: atest CtsSystemUiTestCases:PipNotificationTests
Change-Id: I25205717753b7d0d6c2f9742fb85aed897f86aaa