static/frontend: add search aria-describedby

[essjay05]

Currently the search input aria-label is static and when
focused on by a screen reader, the screen reader does
not read the search tip found below the input. This
change checks the .TipIndex value and if it
matches adds the id=SearchTipContent which the input's
aria-describedby is attached to, thus allowing the visible
tip/description to be read as supplemental info after the
input's existing aria-label.

Before screenshot:
https://screenshot.googleplex.com/AkAmQ69s4R3NsZS

After screenshot:
https://screenshot.googleplex.com/94MwK6bXpSBE6Gr

Fixes b/283297190

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Patch Set 1: Code-Review+2

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