Don't unload shared libraries for null class loader.

If we load a shared library in the boot class loader (null),
then we create the SharedLibrary with a null class loader. We
shouldn't unload this shared library if the class loader decodes to
null since it may still be in use.

Fixes some hangs in gcstress tests.

Bug: 22720414
Change-Id: I52eca4cdba2c5ddd8e3dc7d133d253e5626c5b44