WifiManager: setWifiApEnabled always returns false

The method setWifiApEnabled should not be used.  Introduction of the
start/stopTethering API left this call in an intermediate state, thus
introducing race conditions.  Callers with proper permissions should be
calling ConnectivityManager.startTethering or
WifiManager.startLocalOnlyHotspot.

Note: integration tests are being updated to reflect this change.

Bug: 38436732
Test: frameworks/base/wifi/tests/runtests.sh
Change-Id: I84b9514bf6bb67d733fd1a32492e0e0d485e2cd6