Support multiple sources for prebuilt_etc

Keep the Src attribute for compatibility. The new attribute (Srcs) is
mutually exclusive with Src.

Keep SourceFilePath and OutputFile for compatibility with other modules.
These can be removed in a follow up change.

Bug: 328313691
Test: presubmit
Test: m blueprint_tests
Test: prebuilts/build-tools/build-prebuilts.sh (on build-tools branch)
Change-Id: I5d5b2657715a7180a829c7ed0f501872d561b662

java/sdk_library.go

@@ -30,6 +30,7 @@ import (
 
 	"android/soong/android"
 	"android/soong/dexpreopt"
+	"android/soong/etc"
 )
 
 const (
@@ -3163,10 +3164,12 @@ func (module *sdkLibraryXml) SubDir() string {
 }
 
 // from android.PrebuiltEtcModule
-func (module *sdkLibraryXml) OutputFile() android.OutputPath {
-	return module.outputFilePath
+func (module *sdkLibraryXml) OutputFiles(tag string) (android.Paths, error) {
+	return android.OutputPaths{module.outputFilePath}.Paths(), nil
 }
 
+var _ etc.PrebuiltEtcModule = (*sdkLibraryXml)(nil)
+
 // from android.ApexModule
 func (module *sdkLibraryXml) AvailableFor(what string) bool {
 	return true