Add methods to pause/resume tracking

[klausw]

@AdaRoseCannon suggested this in the TPAC discussion.

Image tracking can have a performance cost, and maybe also other side effects such as enabling camera autofocus which can degrade general tracking performance or stability. If an application only needs image tracking temporarily, i.e. to find a static image and place an anchor there, it should be able to tell the device that it no longer needs image tracking.

(pause/resume tracking could be a no-op on platforms where there's no significant performance cost.)