NFC: Allow apps to specify they're done with tag.

When an application is no longer interested in
communicating with a tag, but the tag is kept in
range of the device for a long amount of time, we
will burn a lot of power keeping the connection
to it alive.

Instead, allow the app to signal it's "done" with
the tag, which allows us to put the controller
in a low-power mode. This will save power in scenarios
such as an NFC tag embedded in a car dock or
wireless charging dock.

Bug: 26426491
Change-Id: I7a0149ede46e0c7610b06ed1776344b772677350