Generate constants for enum values.

Test: Builds successfully, tests pass and statsd works (it seems).

This will allow us to use those constants instead of literals.
The generated code only augmentes the java constant file.
If needed, the same can be done for the C++ file.
Some of the constant names are very long, but this is due to enum value names that are unnecessarily redundant with the enum names, i.e.
enum ENUM_NAME {
  ENUM_NAME_UNKNOWN = 0;
  ENUM_NAME_VALUE1 = 1;
  ENUM_NAME_VALUE2 = 2;
  ...
}
which can be fixed by avoiding the 'ENUM_NAME_' part in the value names above.
So, when possible, we should use shorter value names in stats_events.proto.

Change-Id: I1ad19b86e28d0df0f8c15d4c995d101423cff4c2