make get_apkid() return versionCode as an integer

[eighthave]

versionCode is strictly defined to be an integer, any other type would be an error. So get_apkid() should return versionCode as an integer or fail trying.

[eighthave]

On second thought, since androguard is primarily about analysis rather than correctness, it might make sense to keep this a str, if this field can be abused.

[erev0s]

Thank you @eighthave for this PR.
Indeed versionCode is defined either as int or long and currently the code takes care of that with str(int(value, 16)). The casting to a string, I believe happens so it is aligned with other parts that return strings.
Overall though you are right, it should be treated as an integer, though given how minor this is and that it does not affect the general correctness of the output, it should be fine.