add tip about including android:id for layout state

<p>By default, the system uses the {@link android.os.Bundle} instance state to save information

about each {@link android.view.View} object in your activity layout (such as the text value entered

into an {@link android.widget.EditText} object). So, if your activity instance is destroyed and

recreated, the state of the layout is automatically restored to its previous state. However, your

recreated, the state of the layout is restored to its previous state with no

code required by you. However, your

activity might have more state information that you'd like to restore, such as member variables that

track the user's progress in the activity.</p>

<p>In order for you to add additional data to the saved instance state for your activity, there's an

additional callback method in the activity lifecycle that's not shown in the illustration from

previous lessons. The method is {@link android.app.Activity#onSaveInstanceState

onSaveInstanceState()} and the system calls it when the user is leaving your activity. When the

system calls this method, it passes the {@link android.os.Bundle} object that will be saved in the

event that your activity is destroyed unexpectedly so you can add additional information to it. Then

if the system must recreate the activity instance after it was destroyed, it passes the same {@link

android.os.Bundle} object to your activity's {@link android.app.Activity#onRestoreInstanceState

onRestoreInstanceState()} method and also to your {@link android.app.Activity#onCreate onCreate()}

method.</p>

<p class="note"><strong>Note:</strong> In order for the Android system to restore the state of

the views in your activity, <strong>each view must have a unique ID</strong>, supplied by the

<a href="{@docRoot}reference/android/view/View.html#attr_android:id">{@code

android:id}</a> attribute.</p>

<p>To save additional data about the activity state, you must override

the {@link android.app.Activity#onSaveInstanceState onSaveInstanceState()} callback method.

The system calls this method when the user is leaving your activity

and passes it the {@link android.os.Bundle} object that will be saved in the

event that your activity is destroyed unexpectedly. If

the system must recreate the activity instance later, it passes the same {@link

android.os.Bundle} object to both the {@link android.app.Activity#onRestoreInstanceState

onRestoreInstanceState()} and {@link android.app.Activity#onCreate onCreate()}

methods.</p>

<img src="{@docRoot}images/training/basics/basic-lifecycle-savestate.png" />

<p class="img-caption"><strong>Figure 2.</strong> As the system begins to stop your activity, it