Settings for the screen magnification feature.

1. Screen magnification on the device has to he explicitly enabled
   by the user before allowing a global gesture related interactions.
   The feature toggle setting is in Settings->Accessibility and it
   is a separate screen with instructions how to use the feature
   and a toggle switch.

2. Added a setting in developer options for toggling the magnification
   auto update feature. If this features is on the screen is zoomed
   out or rotation or activity start. Also if the feature is enabled
   the magnified viewport will scroll to show popped up windows.

bug:5587727

Change-Id: Ic45a21d3a3928e2f5aac034751801f090b261488