Overload QtAndroid::startActivity with a callback

The current way of handling activity results by inheriting from QAndroidActivityResultReceiver
is very un-Qt. This patch allows handling activity results by passing a std::function/lambda
expression to the startActivity function.

Change-Id: Ie9dc91cd02dfcdb16a9ad0cb7858387e75159815
Reviewed-by: BogDan Vatra <bogdan@kdab.com>