Android 7.0 &&7.1:
adb shell dumpsys activity activities | findstr mFocusedActivity
Android 8.0 :
adb shell dumpsys activity activities | findstr mResumedActivity
Android 7.0 &&7.1:
adb shell dumpsys activity activities | findstr mFocusedActivity
Android 8.0 :
adb shell dumpsys activity activities | findstr mResumedActivity