JavaBBinder: implement getInterfaceDescriptor

JavaBBinder previously only implemented transact (which works because
it calls into Java and the Java binder will call checkInterface).

However, when ibinderForJavaObject is used, the binder returned will
not properly report its interface descriptor. In this CL,
getInterfaceDescriptor is implemented so that this is reported
correctly.

Bug: 112664205
Test: atest android.binder.cts (fails without this)
Change-Id: Ic8f7d202021688287edafdc27db25fc8ec2f075c