Set reading level scale to display density instead of a fixed minimum.

On a double tap, zoom to either reading level scale or 0.5f more than
current (overview) scale, whichever is greater. This addresses the
scenario for large screen devices where overview and reading level scale
are the same.

Bug: 5165527
Change-Id: I50216d60f98b2904f06742336b9c8c7c512328a6