Read Aloud Tap To Seek

Adds tap to seek for Read Aloud behind a flag.

Uses Boyer Moore to find the string returned by Contextual Search in the distilled Read Aloud text. If a match is found, playback seeks to that point.

TODO: b/325308883 to hide Tap to Search UI when playback is active.

Bug: 305737581, b/326975726
Change-Id: Ic24eb1d786836def9ce919d81209e0ab5a7134f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5287843
Reviewed-by: Basia Zimirska <basiaz@google.com>
Commit-Queue: Andrea Gomez <andreaxg@google.com>
Reviewed-by: Gang Wu <gangwu@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1268181}