Save WifiAp config changes when hotspot is enabled

We need to properly handle asynchronous events when WifiAp for hotspot
is enabled and the user changes the config.  This CL changes the
behavior to:
1 - mark local boolean indicating a restart is needed
2 - trigger the WifiAp to stop
3 - save the new config
4 - when the stop is complete (and the boolean is set), restart the
WifiAp.

Also fix a few style issues in the file.

BUG:27460194
Change-Id: I7e49ab9c49e125017cf83866a396701e440bf383