Allow Singletons to export Make variables

Register any Singletons that implement a MakeVars method as
MakeVarsProviders, and convert the hiddenapi singleton to
use it.

Test: m checkbuild
Change-Id: I6a2044ad34ef46a8b267762ddfeb51aa01d7734d
diff --git a/android/makevars.go b/android/makevars.go
index 2c2fb6f..c011ea6 100644
--- a/android/makevars.go
+++ b/android/makevars.go
@@ -90,6 +90,25 @@
 	makeVarsProviders = append(makeVarsProviders, makeVarsProvider{pctx, provider})
 }
 
+// SingletonMakeVarsProvider is a Singleton with an extra method to provide extra values to be exported to Make.
+type SingletonMakeVarsProvider interface {
+	Singleton
+
+	// MakeVars uses a MakeVarsContext to provide extra values to be exported to Make.
+	MakeVars(ctx MakeVarsContext)
+}
+
+// registerSingletonMakeVarsProvider adds a singleton that implements SingletonMakeVarsProvider to the list of
+// MakeVarsProviders to run.
+func registerSingletonMakeVarsProvider(singleton SingletonMakeVarsProvider) {
+	makeVarsProviders = append(makeVarsProviders, makeVarsProvider{pctx, SingletonmakeVarsProviderAdapter(singleton)})
+}
+
+// SingletonmakeVarsProviderAdapter converts a SingletonMakeVarsProvider to a MakeVarsProvider.
+func SingletonmakeVarsProviderAdapter(singleton SingletonMakeVarsProvider) MakeVarsProvider {
+	return func(ctx MakeVarsContext) { singleton.MakeVars(ctx) }
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 func makeVarsSingletonFunc() Singleton {