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

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https://hg.mozilla.org/integration/gaia-central/rev/4c8c2ddb03c5
Author: gitmai <mri@tid.es>
Desc: Merge pull request #19104 from gitmai/bug-910182-contacts-export-button-cancel

Bug 910182 -[CONTACTS] Export functionality: add missing cancel export f...
r=jmcf

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https://hg.mozilla.org/integration/gaia-central/rev/8b52e12e157c
Author: mai <mri@tid.es>
Desc: Bug 910182 -[CONTACTS] Export functionality: add missing cancel export for any type of export
Bumping manifests a=b2g-bump