ambient: Reduce auto show wait time to 7s

Bug: b/162891584
Test: manual
Change-Id: I912e7942787b0d2994e85a1570eb5bb1a0171578
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2337681Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795194}

ash/ambient/ambient_controller.h

@@ -44,7 +44,7 @@ class ASH_EXPORT AmbientController
       public chromeos::PowerManagerClient::Observer {
  public:
   static constexpr base::TimeDelta kAutoShowWaitTimeInterval =
-      base::TimeDelta::FromSeconds(15);
+      base::TimeDelta::FromSeconds(7);
 
   static void RegisterProfilePrefs(PrefRegistrySimple* registry);