SystemServer: Add Messenger flag

Bug: 315241296
Bug: 321804999
Bug: 288450479
Test: m Bluetooth | adding flag doesn't change test behavior
Change-Id: I0673861e4faa20541198d70212500b314d4ba2f6

flags/system_service.aconfig

@@ -7,6 +7,13 @@ flag {
     bug: "303552318"
 }
 
+flag {
+    name: "system_server_messenger"
+    namespace: "bluetooth"
+    description: "Replace binder call to the system server with a Messenger to enforce thread safety"
+    bug: "321804999"
+}
+
 flag {
     name: "use_new_airplane_mode"
     namespace: "bluetooth"