[Magnifier - 3] Reduce number of calls to PixelCopy

* Magnifier#show() takes snapshots of the content and displays
  them in the Magnifier bitmap
* calling show(...) consecutive times with the same arguments is
  a no-op if already showing (to cater with the miriad of motion
  events produced by touch which end up calling show(...))
* introduced Magnifier#invalidate(...) which, if currently
  showing, forces updating the content using the last configuration
* clamped the start horizontal value of the Rect which delimits the
  content to show in order to avoid distorting the rendering of the
  magnifier content
* fixed invalidating the magnifier (invalidate() does not
  automatically call invalidate(RectF) !)

Bug: 63531115
Bug: 67296158
Test: bit FrameworksCoreTests:android.widget.TextViewActivityTest
Test: bit CtsWidgetTestCases:android.widget.cts.TextViewTest
Test: manual test that shows the magnifier working
Change-Id: I8e53dfb6582d541922fe05b60311658fb07ca880