[Android][IAP] Add IAP to permission mapping table

By this, user can use make_apk.py to package their application
with IAP extension with correct permission, for example:

$ python make_apk.py --name=apkname --package=org.xwalk.extensions.iap
--permissions=iap --extensions=...
Merge pull request #2088 from deqing/iap-permission

[Android] [IAP] Add IAP to permission mapping table