webapps: Add cleanup task when opening up WebappActivity to clean old web apps

Since there is no sure way of knowing when a web app has been removed from
the Home screen, a heuristic has to be used instead.

The heuristic is that if the web app has not been opened in 3 months then it
can probably be safely removed. The full cleanup check is run once a month when
starting a web app so we don't choke the system every time a web app is opened.

BUG=535174

Review URL: https://codereview.chromium.org/1359383002

Cr-Commit-Position: refs/heads/master@{#350579}