Add ability to request unbuffered dispatching.

While in general input batching has a lot of benefits, some apps would prefer
the input system not batch at all. This is typically because they do some of
the batching as part of their own input processing, as is the case with Chrome.

Bug: 12973909
Change-Id: I2e0c2b4c51bacc06f88245c528aa8849e4c4dab2