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

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https://hg.mozilla.org/integration/gaia-central/rev/f4e805ae81c9
Author: Francisco Jordano <arcturus@ardeenelinfierno.com>
Desc: Merge pull request #12077 from arcturus/bug-911075

Bug 911075 - [Contacts] Nothing happens if the user try export contacts as vcard while the sd card is in use

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https://hg.mozilla.org/integration/gaia-central/rev/5f61e1264903
Author: Francisco Jordano <arcturus@ardeenelinfierno.com>
Desc: Bug 911075 - [Contacts] Nothing happens if the user try export contacts as vcard while de sd card is in use

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https://hg.mozilla.org/integration/gaia-central/rev/16c2e7a76f47
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #12341 from cctuan/918874

Bug 918874 - enable software btn as default for mobile without hardware ...

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https://hg.mozilla.org/integration/gaia-central/rev/8ab96d5d8bc1
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 918874 - enable software btn as default for mobile without hardware home btn