Share communal between keyguard blueprint and flexiglass scene

This change moves the existing placeholder communal view in flexiglass
to a common place be shared with keyguard blueprint. This allows us to
develop communal UI in one place while supporting both infrastructures.

Bug: 304359682
Fix: 304359682
Test: atest DefaultCommunalBlueprintTest
Test: verified communal is shown in both keyguard blueprint by
adb shell cmd statusbar blueprint communal, and flexiglass by swiping
left on the lockscreen

Change-Id: If895046ce088d553143108cafdfa7c92df00484c