Rename misleading variable QT_ANDROID_API_VERSION in CMake and qmake

The variable QT_ANDROID_API_VERSION use in https://github.com/qt/qtbase/blob/4656fabd8a6ff46f786cb48c220511ebe43ea300/cmake/QtPlatformAndroid.cmake#L39 (and the qmake counterpart API_TO_USE https://github.com/qt/qtbase/blob/4656fabd8a6ff46f786cb48c220511ebe43ea300/mkspecs/features/android/sdk.prf#L3) are misleadingly named, those two variables refer to the android api level used to build Qt for Android Java code only, where it's needed to use the latest or the max we target.

What To Do? Rename the variables  and rephrase the comment to reflect what was mention above