Update task descriptions on activity opacity changes

Top opaque system bar appearance depends on opacity of the top activity
Therefore, we should respect the settings of the top activity when it
change the opacity of itself.

Bug: 390368429
Test: Caption turns transparent after launching a transparent activity
in a test app.
Test: New tests pass.
Flag: com.android.window.flags.enable_desktop_windowing_mode

Change-Id: If66bbf29c981fb79fb2eb3f9d750651cf8edca9f