Allow save by AddNetworkFragment itself

Before this change, the calling Activity needs to
check result and get WifiConfiguration to save
data by WifiManager API.

This change allows callings app to specify an auto
save extra to save WifiConfiguration by
AddNetworkFragment itself.

Bug: 268411305
Test: manual
Change-Id: I83002c3de3d6726e9be363d5ac057b4ae9e1707e