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

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https://hg.mozilla.org/integration/gaia-central/rev/7012b7d31b28
Author: Julien Wajsberg <felash@gmail.com>
Desc: Merge pull request #12802 from caiolima/bug-815093

Bug 815093 - Compose/share activities returning to caller app
r=julien

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https://hg.mozilla.org/integration/gaia-central/rev/35ee9ca6baea
Author: Caio Lima <ticaiolima@gmail.com>
Desc: Bug 815093 - Compose activities returning to caller app r=julien

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https://hg.mozilla.org/integration/gaia-central/rev/640ae4833ecb
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Merge pull request #12967 from arcturus/bug-925184

Bug 925184 - [B2G][Settings] The user cannot export contacts to the memory card when USB Storage is enabled

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https://hg.mozilla.org/integration/gaia-central/rev/c644d645fa11
Author: Francisco Jordano <arcturus@ardeenelinfierno.com>
Desc: Bug 925184 - [B2G][Settings] The user cannot export contacts to the memory card when USB Storage is enabled

Changed the approach to not to use UMS and settings, but just the sdcard state.

For that added the state 'SHARED' as not available to use for the import/export, since we could have
inconsistencies while using.

Now our utility for sdcard has an extra option to subscribe to changes in the sdcard.