Merge "Add an observer class for transition lifecycles" into tm-qpr-dev am: cbb61207

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/19582591



Change-Id: I2f31e1f8f612904e21ce53e645ce4e187c017bbf
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>