Fix volume slider not showing when casting in the background

- Modify MediaSessionRecord.canHandleVolume() to check system flag and
prevent non system group routes and session records with no routing
sessions from receiving volume evenets.

Bug: 278071711
Test: Manually using 1P and 3P apps on single / group speakers.
Change-Id: I22f53994bd25509c6d6f8c5761304d2dca9c4e8d