win-capture: Fix getting proper UWP window handles

The "main" windows detected for UWP programs are basically to help
sandbox the programs -- they run in the ApplicationFrameHost process and
help reduce the possibility of other programs trying to access the
actual process window, which is a child window.

To bypass this, go through the list of child windows for the
ApplicationFrameHost window, and then find the one that's attached to
a different process; that different process will always be the target,
and will allows us to open the actual process of the UWP program.