Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/4283fcd52197
Author: Ghislain 'Aus' Lacroix <aus@noiseport.org>
Desc: Merge pull request #15914 from nullaus/task-switcher

Cards View Goodness!

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https://hg.mozilla.org/integration/gaia-central/rev/7759d7419f23
Author: Ghislain 'Aus' Lacroix <aus@mozilla.com>
Desc: bug 939809

* added StackManager.snapshot which gets a copy of the array of applications in the stack
* updated CardsView to get the list of and current app from StackManager
* made StackManager ignore the System application explicitly
* minor changes to help with jshint later
* added 'position' getter on stack manager
* added several tests to ensure that the position is current after stack has changed
* moved apps used in cards view test into a variable used to populate running apps as well as the stack
* removed old user ordering code (web os model task manager...) since it's incompatible with our new model
  and the code does not have any tests and it hasn't been enabled in a long time. We're moving on!
* detect cardsview being dismissed and re-enable edge gestures
* created a local help function to enable/disable edge gestures based on events received
* select currently active application card in cards view when cards view is shown
  if no active application, then default to stack position.
* cards view uses 'position' in stack internally, updated code to stop using old 'origin' name to avoid confusion.
* cards view now broadcasts 'newStackPosition' when it's closed and the stack position has changed. newStackPosition contains the position of the application that will be in the top of the stack.
* made edge swipe detector listen to 'cardviewclosed' with new fancy newStackPosition, this also prevents edge gestures from being re-enabled when we go back to the homescreen.
* made stack manager use newStackPosition to move application that was displayed from cards view to top of stack.
* fixed cards view unit tests
* prevent system app from appearing in the application history stack.
* updated 'runningApps' var name to 'stack' in cards view for clarity
* updated 'displayedApp' var name to 'currentPosition' in cards view for clarity
* added getter/setter for position in stack manager, with tests
* added getter/setter for lifecycleEnabled in edge gestures as well as tests for it's usage
* added test to stack manager to ensure that system app does not show up in stack
* alignCurrentCard after we remove one.
* fix gaia ui test.
* use 'position' getter/setter everywhere in StackManager.
* added test to ensure that internal _current position in StackManager doesn't accidentily become a string.

r=etienne
Bumping manifests a=b2g-bump