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Previously, Assistant was visibility was either true || false.
Now, Assistant is one of { kClosed, kHidden, kVisible  }.

These visibility states are:
- kClosed: Assistant is not visible. Launching Assistant will start
           a new session with a clean stage.
- kHidden: Assistant is not visible. Launching Assistant will resume
           a paused session with the stage in its previous state.
- kVisible: Assistant is visible and a session is in progress.

Per product requirement:
- We become kClosed when explicitly dismissed by the user.
- We become kHidden when dismissed by auto-navigation.

Still TODO: Implement timer to auto-close if hidden for 5min.

See bug for demo.

Bug: b:113129833
Change-Id: I2f0f5dba3c2d58d55e5e494b69a93cbb8a667d66
Reviewed-on: https://chromium-review.googlesource.com/1188748
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586398}