Support enabling ICU in Qt for Android (for QStringConverter)

ICU4C is (partially) available since Android 12/NDK 22b (see https://developer.android.com/guide/topics/resources/internationalization#icu4c ).

We currently support down to Android 8.0 (https://doc.qt.io/qt-6/supported-platforms.html#android ) so this cannot be enabled unconditionally yet, but it would be good to allow users to enable it in their own builds.

Use-case
A customer wants to use QStringConverter with non-Unicode codecs. https://doc.qt.io/qt-6/qstringconverter.html#encodingForName says that such codecs would "be accepted by the QStringConverter constructor when Qt is built with ICU, if ICU provides a converter with the given name".