Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/c75b7fead5c7
Author: Ian Liu <iliu@mozilla.com>
Desc: Merge pull request #17060 from ian-liu/system/bug979992_statusbar_is_hidden_while_attention_screen_showing_in_fullscreen_app

Bug 979992 - [Gaia][System][Window Manager] Status bar is missing duringattention screen showing in fullscreen app. r=@etiennesegonzac

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https://hg.mozilla.org/integration/gaia-central/rev/4d02dc21b9b5
Author: ian-liu <iliu@mozilla.com>
Desc: Bug 979992 - [Gaia][System][Window Manager] Status bar is missing during attention screen showing in fullscreen app.
Bumping manifests a=b2g-bump