unlockDialog: Only show count for multiple notifications

In the common case where we only have a single unread notification
from a particular app, the count doesn't add useful information.
Reduce clutter a bit by only showing the notification count if we
have at least two.

GNOME/gnome-shell!997
6 jobs for !997 with less-counts in 7 minutes and 14 seconds (queued for 3 seconds)
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Status Job ID Name Coverage
  Review
passed #591123
check_commit_log

00:00:23

passed #591125
eslint

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passed #591124
js_check

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  Build
passed #591126
build

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  Test
passed #591127
test

00:02:26

passed #591128
test-pot

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