[A11y] Improve setting to pick caret blink rate

The caret blink rate setting is now a slider from "off" to
"fast". A synthetic pref is used to translate from slider
values to the ms blink rate stored in prefs.

Screenshot: screenshot/6dsdTGmvb9JBrf5

Bug: b/259374492
AX-Relnotes: N/A (behind flag)
Test: New, manual
Change-Id: I6e707bcd3d6072dbcb73c2f932f6e717e69d6df8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5463283


Reviewed-by: Wes Okuhara <wesokuhara@google.com>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1289602}