QtAndoidPrivate::activity() should be accompanied by ::context()

The new service support implies that Activity objects are not always suitable. In the case of a service the Service object is set. In general though both classes are derived from Android's Context object. Therefore QAndroidPrivate and QtAndroid should provide accessors to the resulting Context object.

This was already realised on Java class level via QTBUG-54506. However the QtAndroidExtras and QtAndroidPrivate class should have a similar accessor to return the context object.

For more info see https://codereview.qt-project.org/#/c/165469/