Set background drawable on status_bar_container

Put the background drawable on status_bar_container instead of
status_bar because we always want the status bar background to
fill the entire container.

status_bar receives new margins when corner display cutouts are enabled
which may prevent status_bar from taking up the entire
status_bar_container space.

Test: atest SystemUITests
Fixes: 148355955
Change-Id: Ic7e0c0a79908d2a58d4595d239062435253edfda