Add a "Add Network" button to SavedAccessPointsWifiSettings

Bug: 36719359
Test: runtest --path
packages/apps/Settings/tests/unit/src/com/android/settings/wifi/SavedNetworkSettingsTest.java
Test: Open saved networks from WifiSettings. Ensure that exiting and
resuming the activity keeps the "Add Network" button on the bottom of
the network list. This is tested through the cases where networks are
forgotten/saved before resuming, and that pausing/resuming when the dialog
is open or closed does not affect the result.

Change-Id: Ib719a1f6b9468c0f8f44470eeed9144904672cf1