This provides us a path forward with grpc#7211 (hiding
AbstractManagedChannelImplBuilder and AbstractServerImplBuilder) while
providing users a migration path to manage the ABI breakage (grpc#7552). We
do a .class hack so that recompiling avoids the internal class reference
yet the old methods are still available.

Leaving the classes as-is causes javac to compile two versions of each
method, one returning the public class (e.g. ServerBuilder) and one
returning the internal class (e.g., AbstractServerImplBuilder). However,
we rewrite the signature that is used at compile time so that new
compilations will not reference internal-returning methods.

This is intended to be temporary, just to give a migration path. Once we
have given users some time to recompile we will remove this rewriting
and change the generics to use public classes.