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Before TrayBluetoothHelper would get notified of any changes to devices.
This caused problems in busy environments because TrayBluetoothHelper
would get notified hundreds or even thousands of times per second of
devices changes.

This CL changes to a "pull" pattern where TrayBluetootHelper queries
for devices every second. The devices are then cached so that clients
of TrayBluetoothHelper can access them synchronously.

TrayBluetoothHelper queries for devices as long as the state is
kPoweredOn. If the state changes, the device list is cleared.

Bug: 882346

TEST=The following actions should be performed:

1. Turn bluetooth on and off in system tray.
2. Open device list in system menu.
3. Connect to a device.
4. See that the pod feature button in the system tray indicates a
   device is connected when connected to a device.

Change-Id: Id9028546a627f260527aa737f51c032f74a447ec
Reviewed-on: https://chromium-review.googlesource.com/c/1347638
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611069}