Make touchevent timestamp update on every non Stationary touch points

Touch event's timestamp is set from the |first_touch_pointer_event|'s
timestamp. But in the case that this touch point is not changing but
the others changed, this timestamp is not the latest pointerevent's
timestamp.
This CL changes it to use the the latest timestamp of the pointerevents
in |touch_attribute_map|.

Bug: 973545
Change-Id: I08be3f58e886f40eca17d515b36ec3a6424a38ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1705051
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Commit-Queue: Ella Ge <eirage@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685612}