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

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https://hg.mozilla.org/integration/gaia-central/rev/8749f2b7d799
Author: Greg Weng <snowmantw@gmail.com>
Desc: Merge pull request #20736 from snowmantw/issue1027471

Bug 1027471 - [LockScreen][System] Split notification.js into LockScreen...

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https://hg.mozilla.org/integration/gaia-central/rev/02402032a172
Author: Greg Weng <snowmantw@gmail.com>
Desc: Bug 1027471 - [LockScreen][System] Split notification.js into LockScreen and System parts

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https://hg.mozilla.org/integration/gaia-central/rev/eaafe11b8f73
Author: Dominic Kuo <dkuo@mozilla.com>
Desc: Merge pull request #21294 from dominickuo/bug-1009133

Bug 1009133 - [Music] Add hidden style to the player and done icons, r=jimporter

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https://hg.mozilla.org/integration/gaia-central/rev/0f0c6d1db2a7
Author: Dominic Kuo <dkuo@mozilla.com>
Desc: Bug 1009133 - [Music] Add hidden style to the player and done icons
Bumping manifests a=b2g-bump