docs: New section explaining how to set the custom launch color of TV app

NOTE: Cherry picked from:
https://googleplex-android-review.git.corp.google.com/#/c/732032/

Contains updates incorporating all feedback from Andy's original CL.
Updated with suggestion from Andrew.

Bug: 21274505
Change-Id: If43ba81e770a39239108b8fa9731c2b4df3a9ae6
diff --git a/docs/html/training/tv/start/start.jd b/docs/html/training/tv/start/start.jd
index e55e202..743b61b 100644
--- a/docs/html/training/tv/start/start.jd
+++ b/docs/html/training/tv/start/start.jd
@@ -228,6 +228,27 @@
   design guide.
 </p>
 
+<h3 id="transition-color">Change the launcher color</h3>
+
+<p>When a TV app launches, the system displays an animation that resembles an expanding, filled
+  circle. To customize the color of this animation, set the <code>android:colorPrimary</code>
+  attribute of your TV app or activity to a specific color. You should also set two additional
+  transition overlap attributes to <code>true</code>, as shown in the following snippet from a
+  theme resource XML file:</p>
+
+<pre>
+&lt;resources&gt;
+    &lt;style ... &gt;
+      &lt;item name="android:colorPrimary"&gt;&#64;color/primary&lt;/item&gt;
+      &lt;item name="android:windowAllowReturnTransitionOverlap"&gt;true&lt;/item&gt;
+      &lt;item name="android:windowAllowEnterTransitionOverlap"&gt;true&lt;/item&gt;
+    &lt;/style&gt;
+&lt;/resources&gt;
+</pre>
+
+<p>For more information about working with themes and styles, see
+<a href="{@docRoot}guide/topics/ui/themes.html">Styles and Themes</a>.</p>
+
 <h2 id="tv-libraries">Add TV Support Libraries</h3>
 
 <p>