libbinder_ndk: distinguish exception/status_t

Previously here, I had erroneously used EX_* values instead of using
values corresponding to status_t.

This CL defines the set of errors that can be returned from APIs and
constrains the return set to be exactly and only those values.

Exception values are currently not used, but they will be used for
a Status-like object since they are required for communicating with
the SDK (Java). This is also why they were not removed.

Bug: 111445392
Test: ./ndk/runtests.sh
Change-Id: I24bbebae60c167d7e050ee608c24cc1ffc8a9101