This broke the builds for apps not having categories in their appstream
files.

This is currently done on the signing machines as part of the F-Droid
nightly pipeline, but should rather happen as part of the build process
in the future.

Compared to the binary factory script this has a few extensions already:
- Besides recovering information from APKs we can now consume appdata
  files directly, or scan the entire source dir.
- Screenshots from appdata files are downloaded.
- The 'x-test' language is ignored.
- Donation and translation information are added.
- Add links to the source code repository, if we can determine that.

The result is put into a single archive per APK, so we can easily transfer
that to the signing machine via Jenkins alongside the APK.

This matters for libraries in the same repository as the application that
also have an AAR that needs to be integrated, as well as QML plugins. For
this to work we need to consider the build directory as a search prefix,
and produce the exact directory layout there that androiddeployqt expects.

For libraries this is then almost transparent for the application build
system, the only thing that needs to be taken care of manually is putting
the corresponding -android-dependencies.xml file into the right place in
the build dir as well. A macro wrapping that might be an option to
centralize that logic here as well in the future.

For QML plugins this is transparent if you have them set up to work without
installation already anyway, otherwise that setup has to be done for this
to work.

Example: https://invent.kde.org/pim/itinerary/-/merge_requests/28

https://invent.kde.org/frameworks/knotifications/-/merge_requests/12 would
presumably also need this (not tested yet).

When NDK r20+ is used along with Qt5.12, APK generation fails because
of the layout change in newer NDK. This patch introduces a new variable
USE_LLVM, when this is set for older Qt versions, androiddeployqt uses
LLVM's tools.

Also document --android-platform parameter

Also pass along these values to androiddeployqt

Qt adds the Android ABI to the suffix there unconditionally, without also
adjusting CMAKE_FIND_LIBRARY_SUFFIXES accordingly, breaking find_library()
for things built that way. Unfortunately we can't just set this in our
toolchain file, as CMAKE_FIND_LIBRARY_SUFFIXES is overwritten by CMake
after evaluating the toolchain file. So we need to use the variable_watch
hack for this here, thanks to Aleix for the idea.

With this, find_library() works for both suffixed and un-suffixed libraries
again, such as Poppler built with or without Qt support.