Add synchronized lock for ringback playing start/stop request to avoid am: 7b6c9ba0

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/services/Telecomm/+/22304956



Change-Id: I28219902737fb83378c4aebb597c511dde8ba448
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>