mtp: Send events async.

AFT will not read the events endpoint at all
causing every sendEvent() to block. Since
event ordering doesn't really matter, send
events in their own detached thread instead.

Bug: 36802721
Test: Connect AFT, create a folder from device, check not frozen
Change-Id: I4b8aee93c19fa9c73e6b2f34d9794a491b2433e1
(cherry picked from commit d80492b8)