AudioService: Register MASTER_BALANCE/MONO observer for all users

Audioservice register content observer for MASTER_BALANCE/MONO at the time of initialization/boot up. Registration by default is only applicable for current user which in this case is primary user.
So even when audio balance/mono changes for secondary user, the callback
won't be received in audioservice and hence the settings update won't
have any effect on the audio output.

Bug: 339131898
Test: manual
- Validated that audio output balance and mono varies based on the settings in the secondary user
- atest AudioManagerTest
- atest AudioServiceTest

Change-Id: Ifce17d23e0dd0af61d6e27de7c7da629e9480e77