cros: Clean up filenames in //ash/system/date

* TimeView only displays the time, so move into //ash/system/time
* Move ClockObserver to //ash/system/model to be next to clock_model.cc
* Make the filenames match the class names.
* Fix some includes.

Bug: 924744
Test: ash_unittests
Change-Id: I9dbcc157146c8fff019da7c169f00427f32f76ae
Reviewed-on: https://chromium-review.googlesource.com/c/1432612
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625484}
12 files changed