1.Add a button to launch QR code scanner when add a network.
2.Scan button added in AddNetworkFragment / WifiDialog / WifiSettings.
Bug: 120630683
Test: make RunSettingsRoboTests
Change-Id: I340bfa2247e092f586dd90dfea37c355e681ffee
WifiDialog.java can be launched as "full screen mode"
in some use cases. However the way it's done is that
it uses theme to fake the full screen appearance.
It just feels hacky to use a dialog for full screen UI.
So, we created an "AddNetworkFragment" to make this
page can look like a normal fragment.
Also, clean up some useless code about "full screen mode"
of WifiDialog.
Change-Id: Iedd04c6a8e403cbceb872313314e1cba0d514246
Fixes: 111875856
Test: robo test, manual test
