Extract CrOS-Prefs consts from input_method_utils to ..._prefs_consts.

These consts are persisted (as keys or values) in CrOS-Prefs entries
for CrOS 1P input-method-specific settings, so had better be declared
in a centralised manner together with fellow CrOS-Prefs consts
already present in input_method_prefs_consts, for better tracking
and IfChange-ThenChange with cross-language components.

NO_IFTTT=verbatim-move-without-value-changes

Bug: b:464446763
Change-Id: Iffd31fdc65888737b2ba3f84c469e483725c0e58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7279461
Reviewed-by: John Palmer <jopalmer@chromium.org>
Commit-Queue: Bao-Duy Tran <tranbaoduy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1562102}
NOKEYCHECK=True
GitOrigin-RevId: 6eedf6bd691396f747c4cd26d6a5f2c2f045cbb2