[Reader Mode] Add metrics for Reader Mode prompt dismissal

This CL adds the following histograms for Reader Mode:
- DomDistiller.MessageDismissalConditions: to record the user interactions with the message in conjunction with the accessibility setting
- DomDistiller.ReaderModeAccessibilitySettingSelected: to record when the setting is selected/deselected

The CL also updates the accessibility setting summary string.

Bug: 1295032
Change-Id: I1028567ee1a31c8d53b79caf3e0e7d90cca3d869
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3457862
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Reviewed-by: Evan Liu <evliu@google.com>
Commit-Queue: Aishwarya Rajesh <aishwaryarj@google.com>
Cr-Commit-Position: refs/heads/main@{#971967}