Merge "WatchableImpl: add lock when get the mObservers size." into main am: 54fb353080

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/3372057

Change-Id: Ib7a4ab094ac0987147b57a973f772d89a5497549
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>