Reset QNAM's NetworkConfiguration when state changes.

Since QNAM is initialized with defaultConfiguration, we need to
reset the internal configuration used to the current
defaultConfiguration when the state changes and a new configuration
becomes the default.

Task-number: QTBUG-40234
Change-Id: I50f23c62804f29370915eecac2c92301c5f3ead2
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>