audio: add call assistant usage

Add audio usage for call assistant use cases.
Used by apps playing over call uplink path for call
screening or other in call assitant features.
This usage is reserved for system apps with privileged permission
android.Manifest.permission.MODIFY_PHONE_STATE.

Bug: 135213843
Test: make
Change-Id: I7a2278f36f44a474a47dd85d276340da4df460f3