Make sure prebuilt modules are exported before attempting to use them

Fixes an issue where prebuilt module under unexported soong
namespace overrides source modules when flag `prefer` was set.

Test: Create two modules under the same name, use cc_library_shared
      for first one and cc_prebuilt_library_shared with `prefer` set
      to true.
      Then put the prebuilt module under `soong_namespace {}` and
      attempt to compile it without appending the namespace path to
      product namespaces.
      Without this change it should fail to resolve the
      cc_library_shared module even if it should be available.

Change-Id: I506641218e6af94a0b5f048e09e06d83602e50f5

android/prebuilt.go

@@ -339,6 +339,13 @@ func (p *Prebuilt) usePrebuilt(ctx TopDownMutatorContext, source Module, prebuil
 		return false
 	}
 
+	// Skip prebuilt modules under unexported namespaces so that we won't
+	// end up shadowing non-prebuilt module when prebuilt module under same
+	// name happens to have a `Prefer` property set to true.
+	if ctx.Config().KatiEnabled() && !prebuilt.ExportedToMake() {
+		return false
+	}
+
 	// TODO: use p.Properties.Name and ctx.ModuleDir to override preference
 	if Bool(p.properties.Prefer) {
 		return true