Bug 847241 - Make switching to an app feel faster r=timdream

- use the event appopen instead of appwillopen to hide the card switcher; this
  makes the card switcher disappear after showing the app, and thus this is
  nicer.
- disable the transition when we want to remove "immediately" the card switcher
- add some unit tests about the card switcher showing or hiding
(cherry picked from commit 53f9e4f70911d3e083d25730490263a60041f37d)

Conflicts:
	apps/system/js/cards_view.js
	apps/system/test/unit/mock_sleep_menu.js
Bug 847241 - Make switching to an app feel faster

Last-minute follow-up: revert the change to the visibility transition, as it
made the display transition invisible.
(cherry picked from commit cb368d63abd8505b7afa9f4c1bb6a5979bb5f537)