[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