libbinder: Handle transaction failures correctly

Java code expects status_t != OK to be caught at the JNI level in
android_util_Binder.cpp (see signalExceptionForError).  We were
incorrectly mapping this kind of failure to a special exception type
and writing that exception type to parcels.

Instead, refuse to write EX_TRANSACTION_FAILED to a parcel and return
the status value instead.

While here, remove non-trivial constructors to push authors toward the
more explicit factory methods.  Remove getException() and push authors
toward using the simpler getter methods. Fix minor camelCase issues.

Bug: 25615695
Test: system/tools/aidl integration tests still pass

Change-Id: I7cad3ac8ae8300b5ac0b466606f4934d01e503c5