ViewGroup: Split ACTION_CANCEL events during dispatch

Cancel events were previously not split while being dispatched to
children. This means cancel events can be inconsistent with the rest of
the split gesture stream, since the number of pointers and the down time
of the event can change after the event is split.

To keep the stream consistent, always split cancel events when
disptching events to children. We also ensure that we don't drop
cancel events during dispatch.

Bug: 327503168
Test: Unit tests added in follow-up CLs
Flag: EXEMPT bug fix
Change-Id: I7a72506a2b8e70f90efd9ba2e2ec0d12b7fb098d