Load classes for boot image extension.

Previously we actually failed to load and initialize classes
for boot image extensions. Though some classes may have been
resolved by other code paths, extension art files did not
contain the intended classes. This CL fixes that and some
other bugs that were exposed by this class initialization.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I63a1970f7ff45dc37b14ebd24c5a68f3edacd6ef
4 files changed