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: I8cc1dbfd80c7b3c8ca8fe8ea96f9141e2a762dd7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>