[CrOS PhoneHub] Implement TetherController.

TetherController implementation which utilizes MultiDeviceSetupClient
and CrosNetworkConfig in order to interact with Instant Tethering. If
Instant Tethering is user disabled, AttemptConnection() will first
enable the feature via the MultiDeviceSetupClient, then scan for an
eligible phone via CrosNetworkConfig, and finally connect to the phone
via CrosNetworkConfig. If Instant Tethering is enabled, but there is no
visible Tether network, AttemptConnection() will first scan for an
eligible phone via CrosNetworkConfig, and connect to the phone via
CrosNetworkConfig. If Instant Tethering is enabled and there is a
visible Tether Network previously fetched from observing
CrosNetworkConfig, AttemptConnection() will just connect to the phone
via CrosNetworkConfig. Disconnect() disconnects the Tether network
if one exists.

Bug: 1106937
Change-Id: Ibb3eb92483a721cc5be38d1bf8e3ff5f89e0e1f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2441123
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815343}