[WifiSettings] Add Wi-Fi dialog activity

Add a Wi-Fi dialog activity that can be started by setup wizard to
connect to a Wi-Fi access point.

Also refactored mEdit and mModify in WifiConfigController into an
int-enum mMode, with modes view, connect and modify. This is how the
new modes maps to the old flags:

MODE_VIEW     --  mEdit = false, mModify = *
MODE_CONNECT  --  mEdit = true, mModify = false
MODE_MODIFY   --  mEdit = true, mModify = true

Bug: 23426311
Change-Id: I8e2221fd3c42577068e07686dab245dd5888e0ae