Re-implement UserAgent in browser

- Remove calls to SWE's WebSetting API and use the newer implementation of
  the engine. The newer implementation tracks UserAgent per navigation
  entry.
- Allow to override the UserAgent using command line switches.
- Remove debug option to change UserAgent string.
- BrowserConfig class cleanup.

Change-Id: Ib1cc4763e0cbf0207793d1f4e6b1644a9dbb5c08
diff --git a/res/xml/debug_preferences.xml b/res/xml/debug_preferences.xml
index 8e169bd..3ab9b90 100644
--- a/res/xml/debug_preferences.xml
+++ b/res/xml/debug_preferences.xml
@@ -14,7 +14,7 @@
      limitations under the License.
 -->
 
-<PreferenceScreen 
+<PreferenceScreen
         xmlns:android="http://schemas.android.com/apk/res/android" >
 
     <CheckBoxPreference
@@ -28,13 +28,6 @@
         android:title="@string/pref_development_hardware_accel_skia"
         android:enabled="false" />
 
-    <ListPreference
-        android:key="user_agent"
-        android:title="@string/pref_development_uastring"
-        android:entries="@array/pref_development_ua_choices"
-        android:entryValues="@array/pref_development_ua_values"
-        android:defaultValue="0"/>
-
     <CheckBoxPreference
         android:key="enable_visual_indicator"
         android:defaultValue="false"