winrt: Store exit code in pid file

We have to call Exit() to successfully close an application as done in
25dcc90d

. Unfortunately Exit() always
sets the exit code to 1 and this cannot be changed programmatically.

Hence write the exit code into the pid file which is created when
launched via winrtrunner. winrtrunner then fetches the content and
passes the exit code to its callee. This implies that the pidFile is
not deleted by the app itself anymore.

Task-number: QTBUG-38654
Change-Id: Ib9b6ae4a0d61c9bf7e530e984aae3ad6204c39a0
Reviewed-by: Oliver Wolff <oliver.wolff@theqtcompany.com>