Support extended <uses-permissions/> tags

<uses-permission-sdk-23> is a new tag for requesting permissions on android-23 and above. <uses-permission/> and <uses-permission-sdk-23> both can have optional maxSdkVersion values, and these need to be fully supported in order for the Privileged Extension to be able to check the APK permissions against the index.xml permissions. This is needed so that it can prompt the user once, then download and install the APK after that. Its also needed for transparent background updates to check if the permissions have changed in the update.

This gets closer to the complete support, really the full permissions should be stored in the database. Then the only conversion would happen ewhen parsing the XML. Right now, it still stores the old F-Droid permissions names, i.e. without android.permission.. Then those are converted when they are loaded from the DB into an Apk instance.