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: Ic27840b48a7427818ae89b5fe733e5525763d6b0
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>