WifiManager: add new methods to start softap

Add two new methods to start/stop softap in WifiService.  These are
internal calls that allow WifiService and ConnectivityService to
coordinate the switch to and from softap mode.  These calls will not be
callable by apps and are protected by the NETWORK_STACK permission.  The
calls are added in this CL, but are not used at this time.

Bug: 36540346
Test: compiles

Change-Id: Idabdba7f779cbac999ff88d8993b2f4b1f4a1624