Add a Learn More link to the Physical Web opt-in activity

chrome/android/java/res/layout/physical_web_optin.xml

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2015 The Chromium Authors. All rights reserved.
      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file. -->
 
 <merge
     xmlns:android="http://schemas.android.com/apk/res/android" >
 
     <ScrollView
         android:layout_height="match_parent"
@@ skipping 17 matching lines @@
                 android:contentDescription="@null" />
             <TextView
                 android:layout_height="wrap_content"
                 android:layout_width="wrap_content"
                 android:layout_marginTop="10dp"
                 android:layout_marginBottom="20dp"
                 android:text="@string/physical_web_optin_title"
                 android:textSize="24sp"
                 android:textColor="@android:color/black" />
             <TextView
+                android:id="@+id/physical_web_optin_description"
                 android:layout_height="wrap_content"
                 android:layout_width="match_parent"
                 android:text="@string/physical_web_optin_description"
                 android:textSize="16sp"
-                android:textColor="@color/light_normal_color" />
+                android:textColor="@color/light_normal_color"
+                android:textColorLink="@color/light_active_color" />
 
         </LinearLayout>
 
     </ScrollView>
 
     <View
         style="@style/ButtonBarTopDivider"
         android:layout_gravity="bottom"
         android:layout_marginBottom="56dp" />
 
@@ skipping 31 matching lines @@
             android:paddingStart="16dp"
             android:paddingEnd="16dp"
             android:text="@string/enable"
             android:textAllCaps="true"
             android:textColor="@color/light_active_color"
             android:textSize="14sp" />
 
     </LinearLayout>
 
 </merge>