Rootcanal: Clean up disconnected socket devices.

Link layer devices where not properly garbage collected, resulting in
lingering connections that would keep receiving timer ticks. We now add
support for registering close callbacks on a Device object, and we add
support for a Close method on a device.

Device can implement Close to properly close themselves and notify the
listener of closing. The link layer socket can now clean it self up
properly.

Bug: 198300862
Test: cert/run
Change-Id: Ibff03eea836359770937b4ffe57c79bad3682a8f