Make qmake feature for building Android packages

For iOS, qmake can currently generate packages which can be built and deployed from the command line. This script solves a lot of the same issues as we have on Android, and introduces some qmake variables which can easily be relevant on Android as well.

Taking this as a basis, we should make a similar feature for Android, so that existing .pro files can be built into apks from the command line and especially so that iOS projects can also be used for Android builds and the mobile variables apply in both locations.