Fix AudioSource fromExisting assert fail

- AudioSource::set can be called during ctor, during which
there will be no existing refs, causing fromExisting to fail.
- Removing the fromExisting check
- Remove unnecessary promotions before thread creation in
AudioRecord

Bug: 219554785
Test: atest CtsCameraTestCases
atest android.hardware.cts.CameraTest

Change-Id: I8c8615e39a0e7fa931704b5e505a8e703ab75625