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

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https://hg.mozilla.org/integration/gaia-central/rev/57c407d3e94b
Author: Luke Chang <lchang@mozilla.com>
Desc: Merge pull request #17684 from luke-chang/976413_start_stop_activity_window_factory_v2

Bug 976413 - [System2] Add start/stop methods to ActivityWindowFactory, r=alive

========

https://hg.mozilla.org/integration/gaia-central/rev/81ffcfd3f088
Author: Luke Chang <lchang@mozilla.com>
Desc: Bug 976413 - [System2] Add start/stop methods to ActivityWindowFactory

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "7c7d1648ba4e01b47645cb94fa3c05e1f68be1ce", 
+    "revision": "57c407d3e94ba322688b4bd121eda317806ab44e", 
     "repo_path": "/integration/gaia-central"
 }