Opaque background for shutdown UI on TV

The background of the shutdown UI was transparent.
This lead to the text on the shutdown UI being hard
to read, as text from  underneath might shine through.

This change makes the background of the shutdown UI opaque for TVs.

Bug: 152509487
Test: flash build and then factory reset / reboot the device
Change-Id: I620224b9c7b289fbf4db3ffec78007653bea0d71