Fix snapshots for secure windows

First, also draw system bar backgrounds when drawing a fake
snapshot. For that, refactor the drawing into a separate class so
it can be reused. Also enable fake snapshots for secure windows.

Test: com.android.server.wm.TaskSnapshotControllerTest
Test: com.android.server.wm.TaskSnapshotSurfaceTest
Test: Secure activity with resuming delay, make sure system bars
are covered when reopening app.

Bug: 35710126
Change-Id: I2f0ebc7e7acb80015780a4e882f0a472599efa30