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
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index f8383d9..5942ff3 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -633,6 +633,13 @@
 }
 
 flag {
+    name: "screenshot_multidisplay_focus_change"
+    namespace: "systemui"
+    description: "Only capture a single display when screenshotting"
+    bug: "362720389"
+}
+
+flag {
    name: "run_fingerprint_detect_on_dismissible_keyguard"
    namespace: "systemui"
    description: "Run fingerprint detect instead of authenticate if the keyguard is dismissible."