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

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https://hg.mozilla.org/integration/gaia-central/rev/ab71593de598
Author: dwi2 <dwi2@dwi2.com>
Desc: Merge pull request #17420 from dwi2/merge_bubbletea_971498_979212

Bubble-merge 971498 979212

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https://hg.mozilla.org/integration/gaia-central/rev/a8564c770b76
Author: Tzu-Lin Huang <tzhuang@mozilla.com>
Desc: Bug 971498 - [System2] Instantiable HomescreenLauncher

Conflicts:
	apps/system/js/homescreen_launcher.js
	apps/system/test/unit/app_window_manager_test.js
	apps/system/test/unit/bootstrap_test.js
	apps/system/test/unit/homescreen_window_test.js

Bug 979212 - Require MockHomescreenLauncher in unit test of EdgeSwipeDetector

Conflicts:
	apps/system/test/unit/bootstrap_test.js
Bumping manifests a=b2g-bump