AudioService: RttEnabled as atomic boolean

Use AtomicBoolean for "RTT enabled" state instead of locking
on mSettingsLock.

Bug: 287066735
Flag: EXEMPT bug fix
Test: atest com.android.server.audio.AudioServiceTest#testRttEnabled
Change-Id: I61c2185892d868e77745b9be8ab88619cc1aadb8