endSessionDialog: Check for PackageKit before considering updates/upgrades

GNOME Shell is spitting out some errors in the journal due to its attempts
to speak to PackageKit, which is not present on Endless OS, so let's add
some runtime checks to make sure that PackageKit is actually available
before assuming so and using its proxy to decide which kind of UI to
show to the user when ending the session.

#369
4 jobs for !129 with issue369 in 38 minutes and 17 seconds (queued for 6 minutes and 12 seconds)
detached
Status Job ID Name Coverage
  Review
passed #261270
check_commit_log

00:15:30

 
  Build
passed #261271
build

00:11:59

 
  Test
passed #261272
test

00:10:47

passed #261273
test-pot

00:09:52