flutter/engine#7307 changes the engine side of embedded UIView to only
reject gestures when the framework sends a `rejectGesture` message, so
that gesture resolution can done after a touch sequence has ended (see
PR description for flutter/engine#7307 for more details).

This change makes the framework send a `rejectGesture` message to the
engine when a UiKitView rejects a gesture.

I'm planning to land this PR before the engine side change, so right now
it swallows the exception thrown if there is no engine implementation
for `rejectGesture` (which keeps us with the current behavior). After
this change lands I'll land the engine PR, and then clean up the part
that swallows the exception.