inprocess,core: add ManagedChannelBuilder and ServerBuilder factory hiders

* inprocess,core: add ManagedChannelBuilder and ServerBuilder factory hiders

Because the factory for Channels and Servers resides on the builder
itself, it is easy for subclasses to accidentally inherit the
factory.  This causes confusion, because calling a static method on
a specific class may result in a different class.

This change adds hiding static factories to each builder, and a test
to enforce that each subclass hides the factory.  The test lives in
the interop tests, because it has a classpath dependency on all the
existing transports.

Minor note: the test scans the classpath using a Beta Guava API.
The test can be disabled if the API goes away.