[Power ML] Add observers to VideoDetector.

This cl contains the following changes:
- Add video detector observers to mojo VideoDetector in order to be notified
about the video start/end activities.
- Fix a bug to reset suspend reason upon SuspendDone.

Bug: 784232
Change-Id: If626b6971413c3bc78687e3391e671766aa0fbde
Reviewed-on: https://chromium-review.googlesource.com/942649
Reviewed-by: Dan Erat <[email protected]>
Commit-Queue: Jia Meng <[email protected]>
Cr-Commit-Position: refs/heads/master@{#540308}
3 files changed