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

cc/cmakelists.go

@@ -82,7 +82,7 @@ func (c *cmakelistsGeneratorSingleton) GenerateBuildActions(ctx android.Singleto
 func getEnvVariable(name string, ctx android.SingletonContext) string {
 	// Using android.Config.Getenv instead of os.getEnv to guarantee soong will
 	// re-run in case this environment variable changes.
-	return ctx.Config().(android.Config).Getenv(name)
+	return ctx.Config().Getenv(name)
 }
 
 func exists(path string) bool {