HFP: Cleanup bt_hf_callbacks on the jni thread.

For the race condition, the bt_hf_callbacks will be set null on main
thread when doing btif_hf_upstreams_evt on jni thread. Then it will
trigger crash due to btif_hf_upstreams_evt calling a null point.
The patch is cleanup bt_hf_callbacks on the jni thread to avoid the race
condition.

Bug: 140781191
Test: manual, unit test, auto script stress test.

Change-Id: I67400749120aebe13fa3e791f624c8e595d63379
Merged-In: I67400749120aebe13fa3e791f624c8e595d63379