bluetooth: Refactor UnifiedBluetoothDetailedViewController

Changes UnifiedBluetoothDetailedViewController to update the device list
every 500ms while a scan is active.

Also adds a new observer method specifically for when Bluetooth State
changes and changes UnifiedBluetoothDetailedViewController to use this
method to update the list of devices as well.

Bug: 882346
Change-Id: I13ef2555e6c4c451213afb5d405f707d5f4f19a7
Reviewed-on: https://chromium-review.googlesource.com/c/1314069
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605951}