Updating SystemUI visibility events in Adroid Client.

Updating SystemUI visibility events in Adroid Client.

This change is the basis for an upcoming change which will allow the
user to scroll the remote desktop canvas out from under System UI which
would otherwise obscure it.

This change has two main pieces:
1.) I change the 'soft input mode visible' event into a more generic
System UI visible event.  This event will now be used to inform
interested listeners what the offsets are for the LTRB sides of the
screen so the underlying UI can react accordingly.

2.) This change moves some viewport calculation code into the
DesktopCanvas class.  The DesktopCanvas class is aware of the SystemUI
offsets and also handles positioning the canvas so this felt like the
right place to add in the UI ratcheting code later on.

I also fixed up one calculation for Touch mode where the bounds we were
using to constrain the viewport were not taking System UI into account.
This does not change any user perciptible behavior but should prevent
future issues.

BUG=621633