Merge "Fix NPE when re-registerNetworkCallbacks" into udc-dev am: feb922b0ce

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

Change-Id: Ic4fc45344e2d7e458ecb849bfaf03b1e73ee12b1
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>