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/makevars.go b/android/makevars.go
index d323613..00a20f5 100644
--- a/android/makevars.go
+++ b/android/makevars.go
@@ -105,13 +105,11 @@
 }
 
 func (s *makeVarsSingleton) GenerateBuildActions(ctx SingletonContext) {
-	config := ctx.Config().(Config)
-
-	if !config.EmbeddedInMake() {
+	if !ctx.Config().EmbeddedInMake() {
 		return
 	}
 
-	outFile := PathForOutput(ctx, "make_vars"+proptools.String(config.ProductVariables.Make_suffix)+".mk").String()
+	outFile := PathForOutput(ctx, "make_vars"+proptools.String(ctx.Config().ProductVariables.Make_suffix)+".mk").String()
 
 	if ctx.Failed() {
 		return
@@ -120,7 +118,7 @@
 	vars := []makeVarsVariable{}
 	for _, provider := range makeVarsProviders {
 		mctx := &makeVarsContext{
-			config: config,
+			config: ctx.Config(),
 			ctx:    ctx,
 			pctx:   provider.pctx,
 		}