libbinder_ndk: cpp asBinder/isRemote final

These shouldn't need to be overridden by clients, as requested on an
email thread. In the C++ backend, overriding asBinder incorrectly (to
return nullptr) has caused bugs as well.

Bug: N/A
Test: N/A
Change-Id: I305caf7624096bf42633df3f6cf6d28068d864ff