Cleaning in ListWithEditTextHeaderTest.

Switched to ActivityInstrumentationTestCase2.

As far as http://b/issue?id=2446911 is concerned, I was unable
to reproduce a test fail. This was not the case a week ago.
A fix has probably been submitted in the meantime.

Change-Id: Id6e95218a2c9d29b0b4eb051a7a1288f9a9809fd