Add support for creating APKs with Qt6's androiddeployqt

The basic mechanism is very similar to what we do for Qt5, with some legacy cases removed and adapted to Qt6 changes. However it's no longer done implicitly or via toolchain arguments, but is an explicit function applications have to call instead.

@apol this is heavily inspired by your Qt5 code, which doesn't have an explicit license. Are you ok with considering this BSD-2 licensed?