Refactor zoom overview width calculation.

Refactor zoom overview width calculation in FirstLayout and NewPicture;
also taking care of the case when wide viewport is not enabled.

This fixes the setInitialScale Cts test case, which is broken by some
scale related changed for fixed viewport browser.

issue: 3242034
Change-Id: Ie54d089763ee38029ef25e6029884a17c2135f97