Support test fixtures in dexpreopt package

The tests in dexpreopt are unit tests that don't require the use of
test fixtures but there are a number of tests in java and apex packages
that need to configure dexpreopt behavior so this changes adds support
for modifying the dexpreopt configuration in test fixtures.

Bug: 181070625
Test: m nothing
Change-Id: I7b64c9cf0a94cc548c27b55c5a7875be5ff0e18d