Register xml parsing receiver on background thread

One of a11yManagerService's receivers performs xml parsing.
Since this can take a while, the receiver should be registered on the background thread.

Test: atest AccessibilityManagerServiceTest (you'll need to verify via printing or breakpoints that the receiver is on the right thread)
Flag: aconfig manager_avoid_receiver_timeout
Bug: 333890389
Change-Id: I1ee6617fe1a4471d251c7a6d4c52b82a788776b1
diff --git a/services/accessibility/accessibility.aconfig b/services/accessibility/accessibility.aconfig
index 1ba47e4..82579d8 100644
--- a/services/accessibility/accessibility.aconfig
+++ b/services/accessibility/accessibility.aconfig
@@ -128,6 +128,16 @@
 }
 
 flag {
+    name: "manager_avoid_receiver_timeout"
+    namespace: "accessibility"
+    description: "Register receivers on background handler so they have more time to complete"
+    bug: "333890389"
+    metadata {
+        purpose: PURPOSE_BUGFIX
+    }
+}
+
+flag {
     name: "pinch_zoom_zero_min_span"
     namespace: "accessibility"
     description: "Whether to set min span of ScaleGestureDetector to zero."