Make apex_set support ":<apex>" syntax to access the apex file

The apex and prebuilt_apex module types make the output APEX available
using ":<apex>" syntax, e.g. for use as a data file in a test. This
change adds compatible behavior to apex_set to ensure consistent
behavior when building against prebuilts.

Bug: 259958590
Test: m MODULE_BUILD_FROM_SOURCE=false nothing
Change-Id: Ic6f105976cfdd6af4920cc1e11ab0b4ff72e6a6f

apex/prebuilt.go

@@ -915,6 +915,15 @@ func (a *ApexSet) hasSanitizedSource(sanitizer string) bool {
 	return false
 }
 
+func (a *ApexSet) OutputFiles(tag string) (android.Paths, error) {
+	switch tag {
+	case "":
+		return android.Paths{a.outputApex}, nil
+	default:
+		return nil, fmt.Errorf("unsupported module reference tag %q", tag)
+	}
+}
+
 // prebuilt_apex imports an `.apex` file into the build graph as if it was built with apex.
 func apexSetFactory() android.Module {
 	module := &ApexSet{}