service: A2DP source callback to query the preferred codec

In order to apply user's preferred codec while reconnecting, we need a
callback method to talk to Bluetooth stack whether the mandatory codec
has higher priority than others. This change implements a dummy method
in the fluoride service, so is working with the stack.

Bug: 134131114
Bug: 147572898
Test: bluetoothtbd_test
Change-Id: Ib99cf12eba85450526e22723d1509405469c2ebc
Merged-In: Ib99cf12eba85450526e22723d1509405469c2ebc
(cherry picked from commit 3254942410f182533675645392d7f72d14bf7543)