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

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https://hg.mozilla.org/integration/gaia-central/rev/49b157da6c57
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #20479 from timdream/keyboard-layout-two-exposed

Bug 1024873 - Init all layouts exposed in Keyboards, r=rudyl

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https://hg.mozilla.org/integration/gaia-central/rev/283a581928ac
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 1024873 - Init all layouts exposed in Keyboards

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https://hg.mozilla.org/integration/gaia-central/rev/f77f5f4f4ca7
Author: Albert <acperez@tid.es>
Desc: Merge pull request #20338 from acperez/bug-1020152

Bug 1020152 - Generate pre_installed_collections.json in build time. r=yurenju

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https://hg.mozilla.org/integration/gaia-central/rev/b46f5fae2335
Author: Albert <acperez@tid.es>
Desc: Bug 1020152 - Generate pre_installed_collections.json in build time. r=yurenju
Bumping manifests a=b2g-bump