Handle slice broadcasts on BG thread

There's no point for those to run on main thread and cause ANRs - we're
just rebroadcasting broadcasts.

This CL also enhances thread safety for the code running this.

Bug: 334767208
Flag: com.android.systemui.slice_broadcast_relay_in_background
Test: newly updated SliceBroadcastRelayHandler unit test
Change-Id: I11ef3e49458c4e66e4176a5ab64e0328264f889f
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index c979d05..c61002e 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -784,3 +784,13 @@
       purpose: PURPOSE_BUGFIX
     }
 }
+
+flag {
+    name: "slice_broadcast_relay_in_background"
+    namespace: "systemui"
+    description: "Move handling of slice broadcast relay broadcasts to background threads"
+    bug: "334767208"
+    metadata {
+      purpose: PURPOSE_BUGFIX
+    }
+}
\ No newline at end of file