Add placeholder when media control title is blank

When an app posts a media control with no available title, show a
placeholder string with the app name instead

Bug: 274775190
Test: atest MediaDataManagerTest
Change-Id: Ie406c180af48653595e8e222a15b4dda27de2e0e
Merged-In: Ie406c180af48653595e8e222a15b4dda27de2e0e
Merged-In: I1cb6010f19623bd7d5e5bbb5d829b9e4936894b8