Remove background for the icon area in disabled state.

- The icon area background is needed only when the device is
 connected. The icon area background matches the seekbar
 background.
- This CL removes the unnecessary logic of updating the icon area
background color in disconnected or connecting states. For these
states the background should be simply removed.

Flag: EXEMPT refactor
Bug: 387570618
Test: atest SystemUIGoogleRoboRNGTests:MediaOutputAdapterScreenshotTest
Change-Id: I48b593b135361964eb9c54c77afad70b3ee8abe5