handle bad SDK Version values in APKs

Even though it is invalid to have *SdkVersion in AndroidManifest.xml set as
anything but an integer, sometimes people manage to get something in there.
`fdroid update` needs to be able to handle all that.

* https://developer.android.com/guide/topics/manifest/uses-sdk-element#min
* https://gitlab.com/souch/SMSbypass/blob/v0.9/app/src/main/AndroidManifest.xml#L29
* https://gitlab.com/souch/SMSbypass/blob/v0.9/app/src/main/res/values/strings.xml#L27

admin#65