Apply "Dominant Palette A - 800" color to the background of

One-Handed mode

Change the One-Handed mode background color to
Dominant Palette A - 800. (#3C4043)

Bug: 177314967

Test: manual
Test: atest WMShellUnitTests
Change-Id: I68852d362f6f425832d36f3e60c2a41b9e505216