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

webrtc/examples/androidapp/res/values/strings.xml

 <?xml version="1.0" encoding="utf-8"?>
 <resources>
     <string name="app_name" translatable="false">AppRTC</string>
     <string name="settings_name" translatable="false">AppRTC Settings</string>
     <string name="disconnect_call">Disconnect Call</string>
     <string name="room_description">
         Please enter a room name. Room names are shared with everyone, so think
         of something unique and send it to a friend.
     </string>
     <string name="favorites">Favorites</string>
@@ skipping 120 matching lines @@
     <string name="pref_disable_built_in_agc_dlg">Disable hardware AGC.</string>
     <string name="pref_disable_built_in_agc_default">false</string>
     <string name="pref_built_in_agc_not_available">Hardware AGC is not available</string>
 
     <string name="pref_disable_built_in_ns_key">disable_built_in_ns_preference</string>
     <string name="pref_disable_built_in_ns_title">Disable hardware NS.</string>
     <string name="pref_disable_built_in_ns_dlg">Disable hardware NS.</string>
     <string name="pref_disable_built_in_ns_default">false</string>
     <string name="pref_built_in_ns_not_available">Hardware NS is not available</string>
 
+    <string name="pref_enable_level_control_key">enable_level_control_preference</string>
+    <string name="pref_enable_level_control_title">Enable level control.</string>
+    <string name="pref_enable_level_control_default">false</string>
+
     <string name="pref_miscsettings_key">misc_settings_key</string>
     <string name="pref_miscsettings_title">Miscellaneous settings.</string>
 
     <string name="pref_room_server_url_key">room_server_url_preference</string>
     <string name="pref_room_server_url_title">Room server URL.</string>
     <string name="pref_room_server_url_dlg">Enter a room server URL.</string>
     <string name="pref_room_server_url_default" translatable="false">https://appr.tc</string>
 
     <string name="pref_displayhud_key">displayhud_preference</string>
     <string name="pref_displayhud_title">Display call statistics.</string>
     <string name="pref_displayhud_dlg">Display call statistics.</string>
     <string name="pref_displayhud_default" translatable="false">false</string>
 
     <string name="pref_tracing_key">tracing_preference</string>
     <string name="pref_tracing_title">Debug performance tracing.</string>
     <string name="pref_tracing_dlg">Debug performance tracing.</string>
     <string name="pref_tracing_default" translatable="false">false</string>
 </resources>