Create test for screenshots with null snapshots

Snapshots can be null when a layer is offscreen and unparented.
The best way to simulate this in a test is to mirror an offscreen
layer and check if the screenshot path works. Also adds more
informative screenshot deubg logs.

Bug: b/331874495, b/331923495
Test: atest ScreenCapturetest
Change-Id: I4fcd822f00e6427ebaf899756f95e6790b035544