Card unmasking prompt - add accessibility for checkbox tooltip

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

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2014 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.
 -->
 
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content">
@@ skipping 113 matching lines @@
             android:gravity="start"
             android:textSize="12sp"
             android:textColor="@color/input_underline_error_color"
             android:visibility="gone" />
 
         <RelativeLayout
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
             android:layout_marginTop="16dp">
 
-            <!-- TODO(estade): Content description? -->
             <ImageView
                 android:id="@+id/store_locally_tooltip_icon"
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"
                 android:layout_marginEnd="12dp"
                 android:layout_alignParentEnd="true"
                 android:layout_centerVertical="true"
                 android:padding="12dp"
                 android:src="@drawable/help_outline"
-                android:contentDescription="@null" />
+                android:contentDescription="@string/learn_more" />
 
            <CheckBox
                 android:id="@+id/store_locally_checkbox"
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"
                 android:layout_alignParentStart="true"
                 android:layout_toStartOf="@id/store_locally_tooltip_icon"
                 android:layout_centerVertical="true"
                 android:layout_marginStart="17dp"
                 android:layout_marginEnd="10dp"
@@ skipping 17 matching lines @@
         android:gravity="center"
         android:visibility="gone">
 
         <ProgressBar
             android:id="@+id/verification_progress_bar"
             android:layout_width="36dp"
             android:layout_height="36dp"
             android:layout_marginBottom="8dp"
             android:visibility="gone" />
 
-        <!--  TODO(estade): add a real content description for this image. -->
         <ImageView
             android:id="@+id/verification_success"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:src="@drawable/verify_checkmark"
             android:visibility="gone"
             android:contentDescription="@null" />
 
         <TextView
             android:id="@+id/verification_message"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:textColor="?attr/colorAccent"
             android:textSize="20sp" />
     </LinearLayout>
 </RelativeLayout>