This fixes KDEInstallDirsTest.relative_or_absolute_qt unittest.

- We don't need the Threads target workaround anymore, that breaks the
build with Qt6 even.
- The Gradle wrapper shipped with Qt is no longer installed as executable,
so we need to run this in sh explicitly.
- Qt6 uses a different Android Gradle plugin version (not to be confused
with the Gradle version), which we need to make available for the
configure_file() call on the build.gradle file.

With this most Framework modules build against Qt6 here.

Since KDEInstallDirs6 explicitly sets KDE_INSTALL_DIRS_NO_DEPRECATED to
false.