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/env.go

@@ -48,7 +48,7 @@ func EnvSingleton() Singleton {
 type envSingleton struct{}
 
 func (c *envSingleton) GenerateBuildActions(ctx SingletonContext) {
-	envDeps := ctx.Config().(Config).EnvDeps()
+	envDeps := ctx.Config().EnvDeps()
 
 	envFile := PathForOutput(ctx, ".soong.environment")
 	if ctx.Failed() {