[2/n] Add opaque background if task in freeform

Force set an opaque background colour for all freeform tasks to prevent
risk of other floating tasks below from being visible if the window
above is translucent.

Flag: com.android.window.flags.enable_opaque_background_for_transparent_windows
Bug: 397219542
Test: atest WindowDecorationTests
Test: atest DesktopModeStatusTest

Change-Id: Iea4c173681ac6f2a2b4105a91b839328d4a4be72