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
diff --git a/android/paths_test.go b/android/paths_test.go
index 82ae4dc..1e4ba53 100644
--- a/android/paths_test.go
+++ b/android/paths_test.go
@@ -194,7 +194,7 @@
 	return pathtools.MockFs(nil)
 }
 
-func (m moduleInstallPathContextImpl) Config() interface{} {
+func (m moduleInstallPathContextImpl) Config() Config {
 	return m.androidBaseContextImpl.config
 }