A2DP: don't apply codec preference if service disabled

Because the codec preference is saved at upper-layer, it was timing that
JNI callbacks were reset by BT turning off while querying the
preference. In order to prevent NPE, this CL checks whether A2DP is
still enabled, and uses a JNI variable to store those callbacks.

Tag: #stability
Fixes: 182756107
Test: atest net_test_btif
Change-Id: Iacfb9fee0e69a47ca7c7ed7c5fc8b4782b647f84