Update scrubClass() to match CoreTestRunner.

AndroidTestCase.scrubClass() is quite aggressive, and would end up
clearing out final fields like these:

private static final byte[] TEST_DATA = new byte[1];

This change updates the logic to match InstrumentationCoreTestRunner
in the cleanup() method.

Change-Id: Iba455ea35b2628473ce0ccc1431ab47c1caad45c