Support circular magnification frame on circular devices

The magnification viewport expects its boundary to be a rectangular region,
and always draws it as such.  This change causes the indicator to draw as a
circle on devices with circular displays.  This also refactors the width of
the indicator's frame to use a proper dimension resource and updates the
width to 4dip.

Bug:18242438
Change-Id: I1d86647b6d1ef84f5dd506f4141223ec050a79b5