improve QR scanning of repo URI across a range of QR apps

Lots of QR Code scanning apps are quite buggy or lame.  They do not always
respect custom URI schemes like fdroidrepo:// and they often force any URI
to a internal web view.  Using http:// in the QR means worst case, the
user will see the bootstrap index page, where they can click on the
fdroidrepo:// URI in that page.
parse minSdkVersion from AndroidManifest.xml

PackageManager doesn't give us minSdkVersion, so we have to parse it.

fixes #3016 https://dev.guardianproject.info/issues/3016