Merge "TelephonyConnection: Serialize onHold()/onUnhold()" into udc-qpr-dev am: f74ecc88ee

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/services/Telephony/+/23215746

Change-Id: I668e8eaebe753395275d1fb5e5756aa325b39cda
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>