Add a flag for DesktopWallpaperActivity changes.

Add a new flag to mark changes on DesktopWallpaperActivity. Then use
then use this flag to set the launcher alpha to 0 on legacy
RemoteAnimationRunnerCompat.

Fix: 380868195
Test: m & Use back navigation on settings app in desktop mode, observe
that there is no launcher appearing twice
Flag: EXEMPT Bugfix

Change-Id: I2e184e4e78a4a3951f7faa1531a9bc17495ff8a5