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

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https://hg.mozilla.org/integration/gaia-central/rev/a20ae9b6e93b
Author: Min-Zhong "John" Lu <jlu@mozilla.com>
Desc: Merge pull request #25209 from mnjul/bug_1074613_normalize_alt_char_and_overwrite_chars

Bug 1072191, 1074613 - Layout normalization follow-ups. r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/5c1c15e6cc28
Author: John Lu [:mnjul] <jlu@mozilla.com>
Desc: Bug 1072191, 1074613 - Layout normalization follow-ups

- |LayoutLoader._normalizeLayout| is now delegated onto LayoutNormalizer
-- Alternativs chars normalization codes are moved to LayoutNormalizer
- LayoutNormalizer is no longer a singleton (better pattern since it's only used locally)
- textLayoutOverwrites is now normalized
Bumping manifests a=b2g-bump