Introduce BazelStringOrLabelFromProp.

Soong supports string properties, but they are overloaded, and can mean
one of three things:

* path reference
* module reference
* string literal

Bazel has different types: label and string attributes. Thus there needs
to be a way to categorize them correctly in bp2build.

This CL introduces a new function to be used on properties like
apex_key.private_key / apex_key.public_key, as well as
android_app.certificate / apex.certificate.

It is important to disambiguate the prop betenn a string literal
attribute or file/rule target label attribute, so this functions does
just that.  The new attributes are then further handled by their
respective macros (apex_key, android_binary, apex).

Bug: 253557437
Fixes: 253557437
Test: presubmits, new tests

Change-Id: Id8111cdd60d3aabcae7d17fe9da84d0ee3966023
diff --git a/java/app.go b/java/app.go
index bbd9d2d..2a51e10 100755
--- a/java/app.go
+++ b/java/app.go
@@ -1486,22 +1486,8 @@
 	*bazelAapt
 	Deps             bazel.LabelListAttribute
 	Custom_package   *string
-	Certificate      *bazel.Label
-	Certificate_name *string
-}
-
-// ParseCertificateToAttribute splits the certificate prop into a certificate
-// label attribute or a certificate_name string attribute.
-func ParseCertificateToAttribute(ctx android.TopDownMutatorContext, certificate *string) (*string, *bazel.Label) {
-	var certificateLabel *bazel.Label
-	certificateName := proptools.StringDefault(certificate, "")
-	certModule := android.SrcIsModule(certificateName)
-	if certModule != "" {
-		c := android.BazelLabelForModuleDepSingle(ctx, certificateName)
-		certificateLabel = &c
-		certificate = nil
-	}
-	return certificate, certificateLabel
+	Certificate      bazel.LabelAttribute
+	Certificate_name bazel.StringAttribute
 }
 
 // ConvertWithBp2build is used to convert android_app to Bazel.
@@ -1513,7 +1499,8 @@
 
 	aapt := a.convertAaptAttrsWithBp2Build(ctx)
 
-	certificateName, certificate := ParseCertificateToAttribute(ctx, a.overridableAppProperties.Certificate)
+	certificate, certificateName := android.BazelStringOrLabelFromProp(ctx, a.overridableAppProperties.Certificate)
+
 	attrs := &bazelAndroidAppAttributes{
 		commonAttrs,
 		aapt,