DO NOT MERGE Split BackEvent into an internal BackMotionEvent and public BackEvent. am: 2c2265c345

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/20841116

Change-Id: Ia489d2b096b35f7210dad25d74853ee06ad013be
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>