Add a configure flag to enable building Qt for older Android OS versions

Based on:
https://codereview.qt-project.org/c/qt/qtbase/+/558271

"unsupported android version", given that from 6.8 onwards that will be Android 9 and it is not possible to build for older versions like Android 8 after that change.

As suggestion:
Add a configure flag to enable building Qt for older Android OS versions (which might still work even if not supported) in order to bypass the hardcoded limitation for those users who want to build Qt for older versions of Android.

Or add to the Qt for Android documentation that compiling against versions older than <x> is not supported but might work and revert the:
https://codereview.qt-project.org/c/qt/qtbase/+/558271