Add flag a2dp_concurrent_source_sink

Flag to control the code path when both a2dp source
and sink profiles are enabled concurrently.

Bug: 321806163
Bug: 315241296
Test: m (launch raven_car-trunk-userdebug)
Change-Id: Ic2764d7196f36608a768f604f447218b80a7607b

flags/a2dp.aconfig

@@ -6,3 +6,10 @@ flag {
     description: "Enable a2dp offload codec extensibility"
     bug: "308686081"
 }
+
+flag {
+    name: "a2dp_concurrent_source_sink"
+    namespace: "bluetooth"
+    description: "Support for concurrent A2DP source and sink"
+    bug : "321806163"
+}