Rename alpha and refactor the way alpha is set in PageIndicator am: cdadbaa225

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/Launcher3/+/20010024

Change-Id: I233cc6a35ee4ac7d3f3b0d3f8339479e0dd1e15c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>