Add an option to enable level control to Android AppRTC Demo.

webrtc/examples/androidapp/res/xml/preferences.xml

 <?xml version="1.0" encoding="utf-8"?>
 <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">
     <PreferenceCategory
         android:key="@string/pref_videosettings_key"
         android:title="@string/pref_videosettings_title">
 
         <CheckBoxPreference
             android:key="@string/pref_videocall_key"
             android:title="@string/pref_videocall_title"
             android:dialogTitle="@string/pref_videocall_dlg"
@@ skipping 117 matching lines @@
             android:key="@string/pref_disable_built_in_agc_key"
             android:title="@string/pref_disable_built_in_agc_title"
             android:dialogTitle="@string/pref_disable_built_in_agc_dlg"
             android:defaultValue="@string/pref_disable_built_in_agc_default" />
 
         <CheckBoxPreference
             android:key="@string/pref_disable_built_in_ns_key"
             android:title="@string/pref_disable_built_in_ns_title"
             android:dialogTitle="@string/pref_disable_built_in_ns_dlg"
             android:defaultValue="@string/pref_disable_built_in_ns_default" />
+
+        <CheckBoxPreference
+            android:key="@string/pref_enable_level_control_key"
+            android:title="@string/pref_enable_level_control_title"
+            android:defaultValue="@string/pref_enable_level_control_default" />
     </PreferenceCategory>
 
     <PreferenceCategory
         android:key="@string/pref_miscsettings_key"
         android:title="@string/pref_miscsettings_title">
 
         <EditTextPreference
             android:key="@string/pref_room_server_url_key"
             android:title="@string/pref_room_server_url_title"
             android:inputType="text"
             android:defaultValue="@string/pref_room_server_url_default"
             android:dialogTitle="@string/pref_room_server_url_dlg" />
 
         <CheckBoxPreference
             android:key="@string/pref_displayhud_key"
             android:title="@string/pref_displayhud_title"
             android:dialogTitle="@string/pref_displayhud_dlg"
            android:defaultValue="@string/pref_displayhud_default" />
 
         <CheckBoxPreference
             android:key="@string/pref_tracing_key"
             android:title="@string/pref_tracing_title"
             android:dialogTitle="@string/pref_tracing_dlg"
            android:defaultValue="@string/pref_tracing_default" />
     </PreferenceCategory>
 
 </PreferenceScreen>