HFP: Use do_in_jni_thread to avoid btif_transfer_context

* Use do_in_jni_thread and base::Bind() to pass callbacks to JNI thread
* Remove unnecessary code after the refactoring

Bug: 70538124
Test: make, conntect to HFP-enabled car kits
Change-Id: Iea8f19732d40bf40b056fea760ee5cf785a5812b