libbinder_ndk: pointer equality of AIBinder.

This means that there is always a 1-1 correspondance between any object
(remote or local) and its AIBinder in a given process. Two cases:

1). For local objects created by the libbinder_ndk:

    AIBinder_new is called once. Whenever a binder is sent and then
    later received, ABBinderTag is used to identify and recover the
    object immediately.

2). For local objects created by libbinder and all remote objects
regardless of their origin (this is what this CL fixes):

    ABpBinderTag is used to tag the object. This tag holds a weak
    pointer to ABpBinder which is used to recover the object.

Bug: 111445392
Test: ./ndk/runtests.sh

Change-Id: I0d52ed5e356b26a62cfbc8e822f274c878d1112d