[CrOS Bluetooth] Add Disconnect() & Forget() to DeviceOperationHandler.

Update DeviceOperationHandler to implement disconnecting and forgetting
Bluetooth devices. This class relays the Disconnect() and Forget()
calls to device::BluetoothDevice.

Bug: 1010321
Change-Id: I1e7d5666914cc0334571bfe221da7e0c7bbb7715
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3163438
Commit-Queue: Gordon Seto <gordonseto@google.com>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#922244}