Support class lookup for InMemoryDexClassLoader

Setting up InMemoryDexClassLoader with 'null' as parent results in the
class loader not being able to load any boot classpath classes. This is
due to the fact that InMemoryDexClassLoader inherits class lookup from
BaseDexClassLoader which delegates it to ART, but ART rejects
InMemoryDexClassLoader as not supported.

Bug: 120603906
Bug: 120613979
Test: InMemoryDexClassLoaderTest
Change-Id: I3139b1bb343b5fc722bcf06f89a6f6a21a3c7c54