messageTray: Only destroy policy after emitting ::destroy

Destroying the policy invalidates it, so accessing it from a
Source::destroy handler (for example to disconnect signal
handlers) currently results in warnings like:

Object .Gjs_ui_messageTray_NotificationApplicationPolicy
(0x7f8c7c0a64a0), has been already deallocated — impossible
to access it.

GNOME/gnome-shell#2238
8 jobs for !1130 with message-tray-warnings in 6 minutes and 3 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #641991
check_commit_log

00:00:26

passed #641993
eslint

00:01:08

passed #641992
js_check

00:00:43

passed #641995
no_template_check

00:00:30

passed #641994
potfile_check

00:00:42

 
  Build
passed #641996
build

00:03:26

 
  Test
passed #641997
test

00:01:28

passed #641998
test-pot

00:01:28