Respected canUninstall() :P

Summary: Disabled uninstalling of applications which can't be uninstalled. Also, applications which cannot be uninstalled always show that they are installed even if users somehow manually edit the configuration.

Test Plan: Manually edited the URI to uninstall applications which can't be unisntalled.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4741