Fix problems with DexClassLoaderTest.

This fixes a problem with the DexClassLoaderTest where oat files from
previous tests were being cached and reused for later tests. Now each
test uses its own directory for loading-test.dex and friends so they
are treated as separate dex files from test to test.

As part of this fix, the helper functions for the tests were
reorganized.

Bug: 21033982
(cherry picked from commit 5099b54a)

Change-Id: I175184277a585840eca50122cf6898f00823ef3e