Merge "Finish supporting floating-IME alpha in split-screen" into rvc-dev am: 9c9434b3 am: b1e36dba am: 2031024b

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/12130198

Change-Id: I2b6268fb73a52ca004fdb56f8d1aca24f0a181b6