Add fast path to VMClassLoader.findLoadedClass

VMClassLoader.findLoadedClass now calls FindClassInPathClassLoader
as a fast path. Exclusive time results (trace view maps launch):

Before:
nativeFillInStackTrace 1.4%
defineClassNative 1.2%
findLoadedClass 0.2%

After:
nativeFillInStackTrace 0.5%
defineClassNative 0.0%
findLoadedClass 0.9%

Bug: 16828525

Change-Id: I1bde48effcd28529778c00ec0fa0dda4e32026a3