For me adding a transparent ImageView did not help remove the black mask completely. The top and bottom parts of map still showed the black mask while scrolling.
So the solution for it, I found in this answer with a small change.
I added,
android:layout_marginTop="-100dp"
android:layout_marginBottom="-100dp"
to my map fragment since it was vertical scrollview. So my layout now looked this way:
<RelativeLayout
    android:id="@+id/map_layout"
    android:layout_width="match_parent"
    android:layout_height="300dp">
    <fragment
        android:id="@+id/mapview"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginTop="-100dp"
        android:layout_marginBottom="-100dp"
        android:name="com.google.android.gms.maps.MapFragment"/>
    <ImageView
        android:id="@+id/transparent_image"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:src="@color/transparent" />
</RelativeLayout>   
To solve the second part of the question I set requestDisallowInterceptTouchEvent(true) for my main ScrollView. When the user touched the transparent image and moved I disabled the touch on the transparent image for MotionEvent.ACTION_DOWN and MotionEvent.ACTION_MOVE so that map fragment can take Touch Events.
ScrollView mainScrollView = (ScrollView) findViewById(R.id.main_scrollview);
ImageView transparentImageView = (ImageView) findViewById(R.id.transparent_image);
transparentImageView.setOnTouchListener(new View.OnTouchListener() {
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        int action = event.getAction();
        switch (action) {
           case MotionEvent.ACTION_DOWN:
                // Disallow ScrollView to intercept touch events.
                mainScrollView.requestDisallowInterceptTouchEvent(true);
                // Disable touch on transparent view
                return false;
           case MotionEvent.ACTION_UP:
                // Allow ScrollView to intercept touch events.
                mainScrollView.requestDisallowInterceptTouchEvent(false);
                return true;
           case MotionEvent.ACTION_MOVE:
                mainScrollView.requestDisallowInterceptTouchEvent(true);
                return false;
           default: 
                return true;
        }   
    }
});
This worked for me. Hope it helps you..