Add DexClassLoader to the list of supported class loaders

DexClassLoader have the same behavior as PathClassLoader and is still in
use by apps. Add it to the list of supported class loaders so that it can be
recognized during secondary dex file loads.

Bug: 38138251
Bug: 36044779

Test: runtest -x
services/tests/servicestests/src/com/android/server/pm/dex/DexoptUtilsTest.java

(cherry picked from commit d08b3156)

Change-Id: If02081d29f4d8ac917dacd877eb75985ea3895a9