[Magnifier - 5] Auto invalidate and misc

* due to having to make the API public, drop the
  invalidate_by_client strategy in favor of self invalidation
  by polling
* in Magnifier#show(...) stop sending screen coordinates in favor
  of view coordinates
* UI/UX:
** width from 200dp to 164dp
** fixed the shadow depth (elevation)
** fixed the rounded corners (2dp)

Bug: 63531115
Test: bit FrameworksCoreTests:android.widget.TextViewActivityTest
Test: bit CtsWidgetTestCases:android.widget.cts.TextViewTest
Test: manual test that shows the magnifier working
Change-Id: I6451767b0a618c0c4ea396ba3ee3144523dadc57