notify: Dont send refresh complete notification if snap refresh observe is connected

[sergio-costas]

The new Refresh Awareness specification moves all the work of showing refresh progress and notifications into a specific snap. To detect when that snap is available or not, snapd checks if there is a snap with the snap-refresh-observe interface connected. In that case, it presumes that it will manage all the notifications, and won't send any by itself.

Unfortunately, only the "Pending refresh" notification checks this case, while the "Refresh complete" notification doesn't. This results in duplicated notifications.

This patch fixes this.

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[sergio-costas]

@ZeyadYasser Moved the checking code into its own, shared function.