Add summary placeholder for bt preference

Then UI won't be janky(as everything slide down a little bit)

Bug: 63910184
Test: RunSettingsRoboTests
Change-Id: Ie4074694f54af92da52f09d2caaab5490fa73647

src/com/android/settings/bluetooth/BluetoothDeviceUpdater.java

@@ -176,6 +176,7 @@ public abstract class BluetoothDeviceUpdater implements BluetoothCallback {
                     new BluetoothDevicePreference(mPrefContext, cachedDevice,
                             mShowDeviceWithoutNames);
             btPreference.setOnGearClickListener(mDeviceProfilesListener);
+            btPreference.setSummary(R.string.summary_placeholder);
             mPreferenceMap.put(device, btPreference);
             mDevicePreferenceCallback.onDeviceAdded(btPreference);
         }