Switch all screenshot saving paths to ImageExporter

Step toward fully deprecating SaveImageInBackgroundTask. Also removes
shouldShowUiOnExternalDisplay boolean (unused).

Bug: 352308052
Flag: com.android.systemui.screenshot_save_image_exporter
Test: atest, manual test of toast pathway with flag on/off
Change-Id: I038508354c1aeb8a8096b4230108baf194f4381e
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index e2ecda3..61dc17d 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -591,6 +591,16 @@
 }
 
 flag {
+    name: "screenshot_save_image_exporter"
+    namespace: "systemui"
+    description: "Save all screenshots using ImageExporter"
+    bug: "352308052"
+    metadata {
+        purpose: PURPOSE_BUGFIX
+    }
+}
+
+flag {
    name: "run_fingerprint_detect_on_dismissible_keyguard"
    namespace: "systemui"
    description: "Run fingerprint detect instead of authenticate if the keyguard is dismissible."