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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:070eff919c85fd83501e380a92e30caf082e9ffc)
Merged-In: Ie406c180af48653595e8e222a15b4dda27de2e0e
Change-Id: Ie406c180af48653595e8e222a15b4dda27de2e0e