Switch usermap from key to instanceId.

Since keys can change over time, it is better to have a unique field
that maps to its corresponding media data. This will reduce the
updates in current user media data map.

Flag: ACONFIG media_controls_refactor DISABLED
Bug: 326281896
Test: Build.
Test: atest MediaDataFilterImplTest
Test: atest SystemUiRoboTests:MediaFilterRepositoryTest.
Test: atest SystemUiRoboTests:MediaCarouselInteractorTest.
Change-Id: Ie70df444a64d0329c5cb7e9ee343236c8e29ae1c