Export proguard_flags_files from android_librarys

If an android_library has a proguard flag file, that
file should also be used when compiling apps with the
library.

Fixes: 171425221
Test: New unit test in app_test.go, and manually
diff --git a/java/aar.go b/java/aar.go
index f1f6848..157d677 100644
--- a/java/aar.go
+++ b/java/aar.go
@@ -536,6 +536,8 @@
 		ctx.CheckbuildFile(a.aarFile)
 	}
 
+	a.exportedProguardFlagFiles = append(a.exportedProguardFlagFiles,
+		android.PathsForModuleSrc(ctx, a.dexProperties.Optimize.Proguard_flags_files)...)
 	ctx.VisitDirectDeps(func(m android.Module) {
 		if lib, ok := m.(AndroidLibraryDependency); ok && ctx.OtherModuleDependencyTag(m) == staticLibTag {
 			a.exportedProguardFlagFiles = append(a.exportedProguardFlagFiles, lib.ExportedProguardFlagFiles()...)