[Magnifier - 14] Follow finger instead of cursor

This CL makes the magnifier follow the finger rather than the cursor in
text editing. Hence, the magnifier's movement becomes horizontally
smooth, rather than only jumping at the same time with the cursor.
Vertically it remains stable relative to the current line.

Bug: 70508649
Test: bit CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: I7e06a7064f0a413379b30c5e2d11ee85d2ad3720