Adjust actual scale when zoom density changes.

Also some fine tuning of zoom scales.
1. Since overview scale will be used to determine min zoom scale, its own
computation shall be free of min zoom scale.
2. make sure mMaxZoomScale is at lease mMinZoomScale.
3. Use default scale in case of non-overview mode, since it reflects the
current zoom density.

issue: 3494868
Change-Id: I4297878b820e437b706bb7e0f143336ff1b39f82