libpdx: Add support for Status<T> return type from message handlers.

Add support for returning Status<T> from a message handler for a
remote method protocol with return type T. This greatly simplifies
error reporting logic in message handler methods.

Bug: None
Test: mm -j30; tests added to libpdx_uds in a follow up CL
Change-Id: Ib84e1ddc1f1c6a6d57b35ffada06f94a28e8e0cc