AVFoundation: fix mediaStatus and state changes

When both state and mediaStatus are updated at the same time, make sure
both variables are updated before emitting the corresponding signals.
Also, always emit mediaStatusChanged() signals before stateChanged() to
make sure mediaStatus changes are not cancelled by some action
resulting from a state change.

Task-number: QTBUG-49578
Change-Id: I0caea8261120595227834dbac7fed286d125bcab
Reviewed-by: Christian Stromme <christian.stromme@qt.io>