nudge-migration: Migrate dictation nudge to new System Nudge framework

Use AnchoredNudgeManager to create the dictation nudge and update tests
to match this new behavior. Guard this new code under a flag
`IsSystemNudgeMigrationEnabled`.

Migration proposal in https://goto.google.com/system-nudge-v2.

screenshot: https://screenshot.googleplex.com/7Bwn9JSSYT75Lkg

With the new updated design, nudges support larger illustrations instead
of smaller vector icons. For this first iteration the nudge was updated
to be text-only - an image can be added in a subsequent CL.

Bug: b:308674967
Change-Id: Ia10b06872659130b23a75789edc3a4f2607348d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5005465
Reviewed-by: Katie Dektar <katie@chromium.org>
Commit-Queue: Kevin Radtke <kradtke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1221841}