Correct MobileAccessibility.stop() method on iOS

Correct MobileAccessibility.stop() method on iOS, postNotification
requires a string as the second parameter, but we should allow a
notification without a string.

Simplify naming convention for accessibility status events.

closedcaptioningstatusdidchange -> closedcaptioningstatuschanged
guidedaccessstatusdidchange -> guidedaccessstatuschanged
invertcolorsstatusdidchange -> invertcolorsstatuschanged
monoaudiostatusdidchange  -> monoaudiostatuschanged