When checking for an update during startup, open the update.status file with read and write access so repeated update attempts are prevented when there is only read access to the update.status file.
When loading the active-update.xml file after startup, open it with both read and write access so the active-update.xml isn't loaded when there is only read access and the client will still receive manual update notifications.
On Windows, when opening the active-update.xml file with both read and write access fails attempt to fix the update directory permissions.
When checking if it is possible to apply updates, first check for write access to the update directory so OS X no longer always returns true and Windows no longer always returns true when the maintenance service can be used.
Sets security.turn_off_all_security_so_that_viruses_can_take_over_this_computer to true in the app update xpcshell tests so Cu.isInAutomation is true when running the tests.

Differential Revision: https://phabricator.services.mozilla.com/D39601

--HG--
rename : toolkit/mozapps/update/tests/unit_base_updater/marAppApplyUpdateSuccess.js => toolkit/mozapps/update/tests/unit_base_updater/marAppApplyUpdateSkippedWriteAccess_win.js
extra : moz-landing-system : lando