docs: Revised Espresso code snippets

The code snippet showing how to migrate subclasses of
ActivityInstrumentationTestCase2 to Espresso didn't compile. These
changes allow it to run successfully. Also removed obsolete section
describing using Espresso with ActivityInstrumentationTestCase2.

Bug: 27453733
Change-Id: Ib7c7e85d3299337f14be1482f7eb574c9a30dc22