Limit capabilities of a11y gesture dispatch.

Changing the service side to accept descriptions of
motion events, not motion events themselves, so we can
control their creation.

Bug: 30647115
Change-Id: Ia6772a1fc05df91818e3f88959d1e2b4a35fe0cc
(cherry picked from commit a8918f23)
(cherry picked from commit 157f416a)