packagekit: Fix checking of APT::Periodic::Update-Package-Lists

Summary:
BUG: 419055

Currently PackageKitNotifier attempts to check the frequency by running apt-config dump and looking for the Apt::Periodic::Update-Package-Lists property in its output. However, on my system the property name starts with uppercase APT rather than Apt, and ends with a semicolon.

This patch is a defensive, backward-compatible fix. It assumes that under some circumstances the previous code was correct to look for Apt and no ending semicolon. To cover both cases, I edited the regexp to allow for an optional semicolon, and to make the comparison case-insensitive.

Reviewers: Discover Software Store, apol, ngraham

Reviewed By: Discover Software Store, apol

Subscribers: plasma-devel

Tags: Plasma, Discover Software Store

Differential Revision: https://phabricator.kde.org/D28167