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)

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;