MediaSessionService: Fix crash with implicit media button receiver

PendingIntent can be built with implicit intent (i.e. intent without
ComponentName), while using it cause exception.
However, apps can still create and set media button receiver with
implicit intent. Prevent crash when MediaSessionService is notify about
such app's media button receiver.

Bug: 147536687
Test: Build and run
Change-Id: Icfdd45edaa8e25dbdeca3c2180bd234718eef202