Do not consider disabled prebuilt variants.

This relands https://r.android.com/1356826 without changes. It got
reverted as part of b/161315642 but is not related to that issue.

Test: m nothing
Test: `m` with prebuilt SDKs in the manifest
Bug: 151303681
Bug: 161315642
Change-Id: I7af892ae28a8208113e0801b1b8a8204edb61ec8
diff --git a/android/prebuilt.go b/android/prebuilt.go
index 9f4df28..269ad5d 100644
--- a/android/prebuilt.go
+++ b/android/prebuilt.go
@@ -225,7 +225,7 @@
 // PrebuiltSourceDepsMutator adds dependencies to the prebuilt module from the
 // corresponding source module, if one exists for the same variant.
 func PrebuiltSourceDepsMutator(ctx BottomUpMutatorContext) {
-	if m, ok := ctx.Module().(PrebuiltInterface); ok && m.Prebuilt() != nil {
+	if m, ok := ctx.Module().(PrebuiltInterface); ok && m.Enabled() && m.Prebuilt() != nil {
 		p := m.Prebuilt()
 		if !p.properties.PrebuiltRenamedToSource {
 			name := m.base().BaseModuleName()