Tether screen cleanup

- Add a summary placeholder reserving 2 lines for wifi tether pref
- Replace DividerPreference with regular Preference

Change-Id: I2e0c36f27abe2c17625d00d80c816b526f59a6fd
Fix: 37518463
Test: visual
(cherry picked from commit d80117dcea)

res/xml/tether_prefs.xml

@@ -23,7 +23,8 @@
 
     <SwitchPreference
         android:key="enable_wifi_ap"
-        android:title="@string/wifi_tether_checkbox_text" />
+        android:title="@string/wifi_tether_checkbox_text"
+        android:summary="@string/summary_two_lines_placeholder" />
 
     <Preference
         android:key="wifi_ap_ssid_and_security"
@@ -34,7 +35,7 @@
         android:key="enable_bluetooth_tethering"
         android:title="@string/bluetooth_tether_checkbox_text" />
 
-    <com.android.settings.DividerPreference
+    <Preference
         android:key="disabled_on_data_saver"
         android:summary="@string/tether_settings_disabled_on_data_saver"
         android:selectable="false"

src/com/android/settings/TetherSettings.java

@@ -37,7 +37,6 @@ import android.os.Handler;
 import android.os.UserManager;
 import android.support.v14.preference.SwitchPreference;
 import android.support.v7.preference.Preference;
-import android.support.v7.preference.PreferenceScreen;
 import android.util.Log;
 
 import com.android.internal.logging.nano.MetricsProto.MetricsEvent;