Make AaptGroupEntry::getMncName accept mnc00

MNC 00 is a valid network code and is used by
some operators, see http://en.wikipedia.org/wiki/Mobile_Network_Code.

Remove the test that checks if atoi returns zero.
The string is validated to contain only numbers
already.

Also discussed here:
https://code.google.com/p/android/issues/detail?id=3657

Change-Id: I3220d7980abe4067c9e7aa4cbc10b5fd0b8c9ea7