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/variable.go b/android/variable.go
index 13b5abf..c8a672e 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -258,7 +258,7 @@
 		property := "product_variables." + proptools.PropertyNameForField(name)
 
 		// Check that the variable was set for the product
-		val := reflect.ValueOf(mctx.Config().(Config).ProductVariables).FieldByName(name)
+		val := reflect.ValueOf(mctx.Config().ProductVariables).FieldByName(name)
 		if !val.IsValid() || val.Kind() != reflect.Ptr || val.IsNil() {
 			continue
 		}