Experimental: verify cached constructors before use.

Caching constructors can cause problems for dynamically loaded code if
the same class name appears in more than on classloader. Before using
cached constructors, verifty that they come from a valid classloader, i.e.
one that appears in the classloader chain of the current contexts
classloader. Remove ones that do not from the map to allow the associated
classes to be unloaded in case they're no longer in use.

Bug: 21690610
Change-Id: I84f2894cd03a5dc0c33aed9cd710e4a1f6d9515f