Add gravity to ringer drawer background.

The problem arises on the phone with fractional dpi scaling. For
example, in case of scale == 3.5:
 - layout_width="56dp" translates to 196px;
 - layout_width="42dp" + paddingLeft="7dp" + paddingRight="7dp"
   translates to 197px;
Setting background gravity to the layer-list background apparently forces
it occupy 196px instead of 197px in this case.

Flag: EXEMPT bugfix
Fixes: 293320952
Test: manual on phone with scale factor 3.5
Change-Id: I4b13ba3a94bcc6cbd3cc332c63c50949f9053e5d