Add Multi Audio focus

Allows the multiple audio focus that requested with AUDIOFOCUS_GAIN.

Test: atest SettingsProviderTest
Test: Manual - play music on A app
Test: play music on B app    // play music on A app and B app
Test: receive a call         // stop music
Test: end a call             // play music on A app and B app
Test: receive a notification // ducking all music

Bug: b/135749683
Change-Id: I4bdd2cdc5e543674f331fc000bd24fd30499bdc3