[flexiglass] Fixes back/home navigation when occluded.

Before this, back or home navigation when the lockscreen is occluded by
another activity (for example, the camera app when locked) didn't work.

This CL fixes that issue.

The approach is one where SysUiState is hydrated with SYSUI_STATE_STATUS_BAR_KEYGUARD_SHOWING_OCCLUDED when on lockscreen and occluded instead of SYSUI_STATE_STATUS_BAR_KEYGUARD_SHOWING when on lockscreen but not occluded.

For posterity, the command used to figure out the difference betwene
SysUiState flags with Flexiglas on and off was:

$ adb shell dumpsys activity service com.android.systemui/.SystemUIService SysUiState

Fix: 308001302
Test: unit tests added
Test: manually verified that back and home gesture both work when
showing the camera app on top of the locked device
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I707a46b502ad7697985794084b05f35493bcdf19