Adapt Android toolchain file and FindGradle to Qt6
- 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.