Isolate batched input scheduling logic.

Having whether something is currently scheduled get modified in the
actual processing logic and away from the scheduling logic makes it
unnecessarily more difficult to reason about.

In this case, we were only checking whether the Choreographer-aligned
runnable was actually scheduled before we'd consume input. When trying
to unbuffer dispatches so that things were no longer
Choreographer-aligned, however, we'd unschedule the that runnable just
before we tried to consume input and hence just before the check . This
meant we never actually consumed the batches until the next one came in
where we would then consume them immediately.  Worse, if another one
never came in then we'd never actually consume the batch.

By removing the scheduling logic from the processing logic we avoid this
situation. Now only the schedule, unschedule, and actual runnable logic
touch their own scheduled state, so that we don't have to consider the
two pieces of logic together except in isolated places.

Bug: 158540186
Bug: 159239700

Test: manual, using app that injects unbuffered dispatch requests either
      at ACTION_DOWN or first ACTION_MOVE
Change-Id: I7521a5a16ed52afc41261f501128b5498ea0602c