Merge "Set alpha to 0 on attach to avoid flicker at transition start." into udc-dev am: dc472531

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/23252176



Change-Id: I24a6936f045ad6e807fbd756a0aeb2e4989db933
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>