Make Application->isInstalled canonical

Summary: We currently inspect phabricator.uninstalled-applications in two places -- when building a list of getAllInstalledApplications() and when asking an application if it isInstalled(). This diff moves that all to the latter function, meaning we can override it in clever and subtle ways for applications that want to add extra defensiveness about whether they should be considered installed or not (e.g. Releeph, in D4932.)

Test Plan: Load the main Phabricator page, pick an application, uninstall it, notice it is absent, then install it and notice the launch icon reappear.

Reviewers: epriestley, Afaque_Hussain

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5326