libbinder_ndk: rename cpp files like headers

binder_$1.h -> $1.cpp

This change is being made in order to prepare to add more files
representing statuses. Since there will be no AStatus type, it doesn't
make sense to make a file called that. If it was named just status,
then it would stand out.

Bug: 111445392
Test: runtests.sh
Change-Id: I6d27dabe4392af6078a4c8d69545fda8c2c2e1da