fix 1-pane drawer overdraw

Ensure DrawerLayout only has one content view (or else it will double-
draw scrims).

Ensure the drawer itself has an opaque background to enable faster
clipping codepath in DrawerLayout.drawChild(). It must be the drawer
itself and not a child that has a background.

Bug: 8753305
Change-Id: Ib3fd18fe5e6fda5738adc4fd12ea6b09034e78ca
3 files changed