Move user switcher to different thread.

Also add this behavior behind a flag.

Test: switch between users.
Fixes: 284095720
Flag: ACONFIG com.android.systemui.switch_user_on_bg DISABLED

Change-Id: I6ac5117fb496eb78a5de50a9145b048aa047a3bf
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index ab4fe76..eaa6d07 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -241,10 +241,16 @@
 }
 
 flag {
+   name: "switch_user_on_bg"
+   namespace: "systemui"
+   description: "Does user switching on a background thread"
+   bug: "284095720"
+}
+
+flag {
     name: "status_bar_static_inout_indicators"
     namespace: "systemui"
     description: "(Upstream request) Always show the network activity inout indicators and "
         "prefer using alpha to distinguish network activity."
     bug: "310715220"
 }
-