Change MotionPredictor API to only support a single device

Before this CL, the MotionPredictor API was inconvenient to use. The
MotionPredictor.predict returned a list of MotionEvents, with one
MotionEvent per-device.

The goal was to force applications to consider multi-device streams.
However, even if the apps use this API, there's no way they can
currently test this behaviour, since the multi-device feature is not yet
available.

For the multi-device streams feature, the current plan is to provide a
new View callback to get these raw streams. In those streams, the
per-device MotionEvents would continue to look the same. However, the
events may now be interleaved between different devices. For example, a
sequence like this would be possible: DOWN(deviceId=2) ->
DOWN(deviceId=3) -> MOVE(deviceId=2).

That means that the app will likely have to do per-device bookkeeping
anyways. So the app might as well just create a per-device
MotionPredictor object, as well.

Bug: 167946763
Test: (cd frameworks/native/services/inputflinger && atest)
Change-Id: I34b8dc02d4f995146cb3eed33888ae34abde29d6