minui: events: refactor event acquisition

Events are now delivered through a callback mechanism during
a call to ev_dispatch(). This will allow us to extend the events
code to handle other devices/fds, not just input. One such example
is the ability to process uevents.

During initialization, we provide an input callback to ev_init
that gets called when a new event is encountered during dispatch.

ev_get has been removed and replaced with ev_get_input() helper
function that can be called from inside the callback to attempt
to get an input event.

The existing client of ev_get in recovery has been split up such
that the input thread just calls ev_wait(); ev_dispatch(); and
the input_callback handles individual events by using the
ev_get_input() helper.

Change-Id: I24d8e71bd1533876b4ab1ae751ba200fea43c049
Signed-off-by: Dima Zavin <dima@android.com>