Always use placeholder for blank media titles

Removes the behavior where apps targeting U would have their
notification cancelled if the media title was blank, and instead uses
the placeholder title for all apps. Also log these instances with the compat
framework to observe impact on apps.

Bug: 274775190
Test: atest MediaDataManagerTest
Change-Id: I1cb6010f19623bd7d5e5bbb5d829b9e4936894b8