Cosmetic changes to zen mode people & sound/vibration screens

On the people screen, have the footer show up all the time and provide a more relevant message (indicating that the chosen people override any app/etc settings) than the current setting.

On the sound/vibration screen, remove the sub-title since it's a bit confusing what exactly is being turned on or off.

Bug: 190180868
Bug: 215564123
Test: manual
Change-Id: I78d407af350a9f99d26d1e5082b752881bd3f186

res/xml/zen_mode_people_settings.xml

@@ -37,7 +37,6 @@
 
    </PreferenceCategory>
 
-   <!-- Footer that shows if user is put into alarms only or total silence mode by an app -->
    <com.android.settingslib.widget.FooterPreference/>
 
 </PreferenceScreen>

res/xml/zen_mode_sound_vibration_settings.xml

@@ -20,7 +20,6 @@
     android:title="@string/zen_category_exceptions" >
 
     <PreferenceCategory
-        android:title="@string/zen_mode_settings_category"
         android:key="zen_mode_settings_category_sound_vibration">
 
         <!-- Alarms -->