[Wi-Fi] Un-relevant string flash then disappear in WiFi direct page.

The "PEER DEVICES" and "REMEMBERED GROUPS" two strings appear then disappear suddenly.

Solution: Set those two PreferenceGroups, which with empty preference
child, invisible before showing.

Bug: 141830944
Test: 1.Manual test. 2.Modify isAvailable() test case for make sure the
initial visible of empty group is invisible.
Change-Id: Ifb074ac758196096ca63b6f7fa1285f8148d98d4

src/com/android/settings/wifi/p2p/P2pCategoryPreferenceController.java

@@ -36,13 +36,13 @@ public abstract class P2pCategoryPreferenceController extends AbstractPreference
 
     @Override
     public boolean isAvailable() {
-        return true;
+        return mCategory.getPreferenceCount() > 0;
     }
 
     @Override
     public void displayPreference(PreferenceScreen screen) {
-        super.displayPreference(screen);
         mCategory = screen.findPreference(getPreferenceKey());
+        super.displayPreference(screen);
     }
 
     public void removeAllChildren() {