Add a background to UMO in Glanceable Hub

This change adds a background to the UMO in the Glanceable Hub ensuring
the settings icon that appears when swiping horizontally has enough
color contrast.

Fix: 378797118
Test: tested manually; see videos in bug
Flag: EXEMPT bug fix
Change-Id: I2e5f178fd466b12a7fb9e237f45cfbd34d82e777