Fix to bottom bar behavior

- Add a new extra for the bottom bar buttons
- Use the above to distinguish between old and new API calls so that
  all permutations of support lib and chrome versions work
- Add accesibility support for bottom bar
- Have the bottom bar pending intents sent back with the current url

BUG=573683

Review URL: https://codereview.chromium.org/1591053003

Cr-Commit-Position: refs/heads/master@{#369907}
4 files changed