interface class provides TRANSACTION_<methodname> consts

Like the Java and the rust backends, cpp and cpp_ndk backends now also
provide transaction codes as TRANSACTION_<method> consts in the
interface class.

Bug: 174187767
Test: aidl_unittests
Change-Id: Ic6f15db21e1af0e03cb690b0e6488d6f697dc080
7 files changed