R1:7487c5740178

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…