Clean up JNI

The callbacks for HeadsetClientConnectionService's NativeInterface was
failing when a non static function, declared in C++, had a matching
method in the corresponding Java file that was declared as static. This
caused a signature mismatch due to the class being a different InstanceOf.

Bug: 34334230
Change-Id: Ib3d18d1615190d5835f72e00bc4d28adfa86a48e