Ignore classes on the classpath when generating stubs

This prevents metalava from generating stubs that reference classes
which are provided on the classpath. That ensures that removing hidden
classes from the sources does not result in references to those removed
classes from being added to the generated stubs, e.g. in the imports.

Specifically needed to prevent the StrictMode stubs class from
referencing dalvik.system.CloseGuard when that is removed from the
sourcepath as a result of switching the stub generation from using
libcore implementation classes (which contain CloseGuard) to
generated stubs (which does not contain CloseGuard).

Bug: 142113521
Test: m checkbuild
Change-Id: Ib4b87fec6549b69cc69820bc8d8b33f8c4e8535c