Dictation: Add hints section to bubble UI

This change adds a hint section to the Dictation bubble UI. Under the
hood, we separate the DictationBubbleView into two helper views. The
first is called TopRowView, which contains the previous implementation
of the bubble UI (introduced in the upstream change). The second is
called HintView (added in this change), which is responsible for
displaying hints. Both classes are kept as implementation details of
DictationBubbleView and implemented in an anonymous namespace.

Please see the following link for UX mocks:
https://docs.google.com/presentation/d/1Uz_tSJKMJFPjoUTsCBtSjYs0XU3_SFjgHABXAgEelSo/edit#slide=id.gbdf663abd1_0_103

AX-Relnotes: N/A
Bug: 1289858
Change-Id: I1428424d6d0cc98b3a22c3796195762a5dbff2a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3408082


Reviewed-by: Anastasia Helfinstein <anastasi@google.com>
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/main@{#963294}