WifiManager: update API for LOHS

Add a return code for disallowed tethering for the user.  Also updated
call to startLocalOnlyHotspot in WifiServiceImpl to return a code
instead of a config.  This allows us to return different failure modes
to the application instead of assuming an incompatible mode error.

Also updated method name to retrieve the wifi config.

Bug: 37073685
Test: frameworks/base/wifi/tests/runtests.sh
Test: frameworks/opt/net/wifi/tests/wifitests/runtests.sh
Change-Id: Iaa442f7062145dbfbdc8e26ae5479d14307addf8