[Android] Change incognito toolbar color back to #505050

The toolbar color was changed in https://codereview.chromium.org/2809923003
for Chrome Home. The specified color, #424242, would have required
updating tablet tab strip assets to a darker grey, which violated
accessibility standards. Reverting to the old color.

BUG=716351,695973

Review-Url: https://codereview.chromium.org/2859933004
Cr-Commit-Position: refs/heads/master@{#469394}

chrome/android/java/res/values/colors.xml

@@ -143,7 +143,7 @@
     <color name="progress_bar_background_white">@color/black_alpha_30</color>
 
     <!-- Theme colors. Also used for toolbar background -->
-    <color name="incognito_primary_color">#424242</color>
+    <color name="incognito_primary_color">#505050</color>
 
     <!-- LocationBar colors -->
     <color name="locationbar_dark_hint_text">#969696</color>