Fix installing prebuilts with no source module

Prebuilts that were not overriding a source module would never
get UsePrebuilt set, which would cause SkipInstall() to be called,
and prevent the module from being exported to make.

Test: prebuilt_test.go
Change-Id: Ib09300efff26f09245331e9919ae2db82b0c3625
diff --git a/android/prebuilt.go b/android/prebuilt.go
index 006b66f..772df7a 100644
--- a/android/prebuilt.go
+++ b/android/prebuilt.go
@@ -76,10 +76,15 @@
 	}
 }
 
-// PrebuiltSelectModuleMutator marks prebuilts that are overriding source modules, and disables
-// installing the source module.
+// PrebuiltSelectModuleMutator marks prebuilts that are used, either overriding source modules or
+// because the source module doesn't exist.  It also disables installing overridden source modules.
 func PrebuiltSelectModuleMutator(ctx TopDownMutatorContext) {
-	if s, ok := ctx.Module().(Module); ok {
+	if m, ok := ctx.Module().(PrebuiltInterface); ok && m.Prebuilt() != nil {
+		p := m.Prebuilt()
+		if !p.Properties.SourceExists {
+			p.Properties.UsePrebuilt = p.usePrebuilt(ctx, nil)
+		}
+	} else if s, ok := ctx.Module().(Module); ok {
 		ctx.VisitDirectDeps(func(m blueprint.Module) {
 			if ctx.OtherModuleDependencyTag(m) == prebuiltDepTag {
 				p := m.(PrebuiltInterface).Prebuilt()
@@ -121,5 +126,5 @@
 		return true
 	}
 
-	return !source.Enabled()
+	return source == nil || !source.Enabled()
 }