Fill task snapshot with background color

Make sure to fill the portions that are not covered by the
snapshot are filled with the task background color.

Also fix an issue where the starting window was removed across
configuration changes.

Test: runtest frameworks-services -c
com.android.server.wm.TaskSnapshotSurfaceTest
Bug: 31339431
Change-Id: I2451be87aff79b337015ab4bba72cfa03c0d3582