Make *Context.Config return a Config instead of a interface{}

In Soong, a Config() method will always return a Config.  Make
ModuleContext, SingletonContext, TopDownMutatorContext and
BottomUpMutatorContext's Config() methods explictly return
a Config to avoid having to type-assert everywhere.  Overriding
the Config method requires duplicating the list of methods in
blueprint.BaseModuleContext and blueprint.BottomUpMutatorContext,
following the same pattern used by the other *Contexts.

Config() obsoletes the AConfig() method used in some places, which
will be cleaned up in the next patch.

Test: m checkbuild
Change-Id: Ibe21efde933959811d52443496967ab8ce71215e

android/package_ctx.go

@@ -47,7 +47,7 @@ type configErrorWrapper struct {
 var _ PathContext = &configErrorWrapper{}
 var _ errorfContext = &configErrorWrapper{}
 
-func (e *configErrorWrapper) Config() interface{} {
+func (e *configErrorWrapper) Config() Config {
 	return e.config
 }
 func (e *configErrorWrapper) Errorf(format string, args ...interface{}) {