window-tracker: Fix pid based window/app association

The lookup table used by get_app_from_window_pid contained only pids of
apps launched by gnome-shell itself, but not pids of apps running before
gnome-shell was (re-)started. Also the pids in that table might not even
be the pid of the process that is actually showing the window if wrapper
scripts are used.

Instead use shell_window_tracker_get_app_from_pid which uses the pids
from the windows themselves.

This removes the only use of launched_pid_to_app which will be removed
in the following commit.

This fixes: https://bugzilla.gnome.org/show_bug.cgi?id=736527
window-tracker: remove now unused launched_pid_to_app

https://bugzilla.gnome.org/show_bug.cgi?id=736527