Accurately track the sequence numbers of batched events.

Instead of sending finished signals immediately when appending to
a batch, record the chain of sequence numbers that were part of
the batch and then send finished signals all at once when done.
This change helps the dispatcher keep track of the true state
of the application and can improve ANR detection slightly.

This is part of a series of changes to improve input system pipelining.

Bug: 5963420
Change-Id: I463c2221e2aa8fdf1c3d670c18e39e59ab69b0db