Outdated Android docs still refer to QtAndroid::androidActivity()

https://doc.qt.io/qt-6/android-services.html#start-a-service-on-demand refers to QtAndroid::androidActivity()

Problem 1: The "QtAndroid" namespace no longer exists in Qt 6

It used to be at https://doc.qt.io/qt-5/qtandroid.html#androidActivity

Some compatibility enums/functions are currently offered under "QtAndroidPrivate": https://doc.qt.io/qt-6/qtandroidprivate.html

Problem 2: The "androidActivity()" function no longer exists in Qt 6

Both https://doc.qt.io/qt-6/android-services.html#start-a-service-on-demand and https://doc.qt.io/qt-6/qtandroidprivate.html still refer to "androidActivity()". This function no longer exists.

Changes needed

• The reference to QtAndroid::androidActivity() in https://doc.qt.io/qt-6/android-services.html#start-a-service-on-demand needs to be updated.
• The references to QtAndroid::androidActivity() in https://doc.qt.io/qt-6/qtandroidprivate.html need to be updated.
• https://doc.qt.io/qt-6/extras-changes-qt6.html#changes-to-qt-android-extras needs to explain what options users have for porting https://doc.qt.io/qt-5/android-services.html to Qt 6.