Make classes visibly initialized in bigger batches.

Change the batch size from 16 to 48. This should make app
startup faster for apps that initialize a lot of classes.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 38383823
Change-Id: If6387dab27887b7380e30ee6e8b3376158ff04de