Move asnyc call audio tasks to single thread. am: 7733488114

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/services/Telecomm/+/23917454

Change-Id: I4eabeb200abb25ebcc155f1b8bf2f79c2119ed9b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>