Merge "Fix NPE when re-registerNetworkCallbacks" into udc-dev am: feb922b0 am: cfe7cf29 am: 0d399e96

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/22598874



Change-Id: I8d91253caeeadeb66ed8ee66696bccd4b8d7be63
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>