[Magnifier-16] Clamp to surface against distortion

Previously, we were only making sure that the magnified content belongs
to the view the magnifier is attached to. However, when the view was
laid out partially outside the screen, we would pixel copy from outside
the surface the view is attached to. This would lead to the user seeing
a distorted content in the magnifier, in cases when the magnified view
lies outside the screen. This CL addresses this issue, by clamping the
pixel copy coordinates inside the surface we copy the magnifier content
from.

Bug: 72039853
Bug: 63531115
Test: bit CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: Iddab05c98b615259938e0d3a3320b98b3b13b246