ozone: evdev: Use kernel timestamp from input events for KeyEvents

ozone: evdev: Use kernel timestamp from input events for KeyEvents

The input events come from the kernel with a timestamp generated with
CLOCK_MONOTONIC so populate the time_stamp field of the Event with this
event time stamp (in lieu of using EventTimeForNow()).

For key repeats use EventTimeForNow() which also uses CLOCK_MONOTONIC
via TimeTicks::Now().

This change introduces a new constructor to KeyEvent which allows the
provision of a time_stamp and a new helper function to convert
an input_event's time to a base::TimeDelta. This helper is used in the
TouchEventConvertEvdev which was making the same calculation.

BUG=450341
TEST=Interactive testing (in particular key repeat) on link_freon and
events_unittests passes

Review URL: https://codereview.chromium.org/864293005

Cr-Commit-Position: refs/heads/master@{#315544}