Clean up the build.gradle(.kts) files in Qt Quick for Android examples

Why?

The files have some functionality and definitions that were added by early versions of the Qt Android Studio Plugin, and some that are not needed anymore. Cleaning these would make everything easier to read for the customer.

What?

Remove useLegacyPackaging in Qt 6.9, remove sourceSets from all versions, remove fileTree from all versions. The Kotlin based examples look better, but check those too.