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This reduces the height of the onboarding dialog, and updates spacing
between other elements to fit the new height as well as to come
inline with the spec.

Screenshots:
  https://screenshot.googleplex.com/4KC2SMGEsQDDcZZ.png
  https://screenshot.googleplex.com/8wWbvuyZS5iS5YJ.png
  https://screenshot.googleplex.com/8v8wrjAfj2cMH5M.png

Spec:
  https://carbon.googleplex.com/cros-ux/pages/nearby-share/onboarding-flow

Bug: b:168841167
Change-Id: I311ca41b8c9269ba1c66ea1f86147894329e09d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530284
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Reviewed-by: James Vecore <vecore@google.com>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826902}