AudioSystem: Ensure onNewService callback is complete before returning service

A race condition with two threads starting at identical times
obtaining the AudioFlinger service the very first time may result
in an AudioTrack creation failure because the second thread continues
before the onNewService callback is complete.

Here the second thread will wait for the onNewService to complete.

Flag: EXEMPT bugfix
Test: atest CtsMediaAudioTestCases
Test: atest WvtsDeviceTestCases:com.google.android.wvts.MediaDrmParameterizedTests#testResourceRatingTierNumKeys[L1] --iterations=100 -- --abi arm64-v8a
Bug: 402768417
Change-Id: Id011af3bcc0ff701beb7e986e415913260606669