[Magnifier-36] Fix content clamping inside view

We try to never display in the magnifier content that does not belong to
the magnified view. If the magnified view has one or more scrollable
containers, these have to be considered in order to find out the visible
portion of the view which is not masked by scrollable containers.

The previous logic for computing the visible region was wrong when one
of the containers was a ViewPager, whose getScrollX() returns the scroll
relative to all pages, rather than the currently visible one as the
logic was expecting. This CL replaces the old logic with a
View#getGlobalVisibleRect().

Bug: 74359490
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: Ib6b63a35436aa691f29c13a0789688f23bfca9f1