ambient: Change the photo view animation time

Change the transition animation duration to 500 ms.

Bug: b/162782654
Test: manual
Change-Id: Ic7ee30843bc04c4666a29e56ff859343acb29b73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2337376Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794735}

ash/ambient/ambient_constants.h

@@ -12,7 +12,7 @@ namespace ash {
 // Duration of the slide show animation. Also used as |delay| in posted task to
 // download images.
 constexpr base::TimeDelta kAnimationDuration =
-    base::TimeDelta::FromMilliseconds(250);
+    base::TimeDelta::FromMilliseconds(500);
 
 // The default interval to refresh photos.
 // TODO(b/139953713): Change to a correct time interval.