reader/android: adds zoom capability
reader/android: adds zoom capability

A first pass at zooming into the PDF using:

    window.client.zoom(<value>)

The initial scale is set based on the document dimensions and viewport width.
The value passed into client.zoom(...) can be changed in the future to match any
value and constraints required by the Android code.

This commit closes #44, any future requirements around the PDF web view should
have their own issues created.

Change-Id: Ibc4ca1261bd784b2665e2e98816de7fb59b7f9fc

vanadium/reader,vanadium/reader,vanadium/reader,jxson/reader,jxson/reader,jxson/reader,jxson/reader