[Wi-Fi DPP] Fix UI defects

1. Set scan icon minWidth 64dp instead of 48dp
2. Show finish button after Wi-Fi Easy Connect handshake successfully

Bug: 129666609
Bug: 135490433
Test: manual visual
Change-Id: Ifcdbe61b314a9cce3043965e9e5b67406c4616f9

res/layout/wifi_button_preference_widget.xml

@@ -19,7 +19,7 @@
            android:id="@+id/button_icon"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
-           android:minWidth="@dimen/min_tap_target_size"
+           android:minWidth="64dp"
            android:minHeight="@dimen/min_tap_target_size"
            android:layout_gravity="center"
            android:background="?android:attr/selectableItemBackground"/>

src/com/android/settings/wifi/dpp/WifiDppAddDeviceFragment.java

@@ -92,6 +92,7 @@ public class WifiDppAddDeviceFragment extends WifiDppQrCodeBaseFragment {
             activity.setResult(Activity.RESULT_OK);
             activity.finish();
         });
+        mRightButton.setVisibility(View.VISIBLE);
 
         if (!isConfigurationChange) {
             mLatestStatusCode = WifiDppUtils.EASY_CONNECT_EVENT_SUCCESS;