Screenshot a single display per invocation

Actual per-source display determination left as TODOs, but this puts the
structure in place under a flag.

Bug: 362720389
Test: atest TakeScreenshotExecutorTest
Flag: com.android.systemui.screenshot_multidisplay_focus_change

Change-Id: I4f0b4f21b1f93e02ccb14546157c79ffad685397
3 files changed