Improve support for content URIs on Android

In 2c60844badd8fb07dd4844926345c924679f20fb, we added support for the native file dialog on Android. Since this file dialog returns content URIs and not local file paths, a file engine was added to support the scheme in 7d2d1eb9e051ad01dc5c5c3053c58364885bdc07, in order for the content URIs to be usable in all the relevant locations in Qt.

However, since content URIs are not necessarily local file paths, no support was added for resolving the actual file name and path of the local file it represents. But for the paths returned by the file dialog, this would be case, so a useful addition would be if QFile returned the correct file name and path in the case where the URI did in fact represent a local file.

The URIs that do not represent files would still return nothing from these functions, which is a bit confusing of course, but a side effect of the fact that Qt is designed to assume all content is provided through a traditional file system, and therefore QFile is at the core of everything.