Generate constants for enum values in cpp header

This is the C++ counterpart of the enum generation for Java done in
ag/I1ad19b86e28d0df0f8c15d4c995d101423cff4c2

Bug: 120990429
Test: builds successfully, logged one of atoms with an enum in C++ code and verified statsd logcat

Change-Id: I2c1452c0590aecdef0dbede6d88014c748e6509e