Bumping gaia.json for 4 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/c8716b0dcb8c
Author: EragonJ <eragonj@eragonj.me>
Desc: Merge pull request #14836 from EragonJ/bug-948847

Bug 948847 - [Gaia] add AirplaneModeHelper to make sure settings app and system app will not do the same thing

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https://hg.mozilla.org/integration/gaia-central/rev/de9595bd9188
Author: EragonJ <eragonj@eragonj.me>
Desc: Bug 948847 - [Gaia] add AirplaneModeHelper

- other apps can use AirplaneModeHelper to know current airplane status
- add a lot of related unit tests
- add two mozSettings key for airplane mode use

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https://hg.mozilla.org/integration/gaia-central/rev/05057baa931c
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #15191 from MBRSL/bug-948858-multiSIM

Bug 948858 - [UITest][HW] modify radio test for multi-SIM card device, r=gasolin

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https://hg.mozilla.org/integration/gaia-central/rev/dfad6bd16ae9
Author: Tom Jao <tjao@mozilla.com>
Desc: Bug 948858 - [UITest][HW] modify radio test for multi-SIM card device

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "7f3fb38df1eeb0e22f308f8443640f2738425e87", 
+    "revision": "c8716b0dcb8cc8946903355c3533481f967b52d3", 
     "repo_path": "/integration/gaia-central"
 }