Fix se_policy_conf file output stem

OutputFileProducer interface has been returning "conf", not the
designated stem.

Test: try including se_policy_conf module as other module's srcs
Change-Id: I17de5e10ed9bd1d45dc9a8b1be11ea6f5290c179
diff --git a/build/soong/policy.go b/build/soong/policy.go
index 4becbc1..390c439 100644
--- a/build/soong/policy.go
+++ b/build/soong/policy.go
@@ -200,7 +200,7 @@
 }
 
 func (c *policyConf) transformPolicyToConf(ctx android.ModuleContext) android.OutputPath {
-	conf := android.PathForModuleOut(ctx, "conf").OutputPath
+	conf := android.PathForModuleOut(ctx, c.stem()).OutputPath
 	rule := android.NewRuleBuilder(pctx, ctx)
 
 	srcs := android.PathsForModuleSrc(ctx, c.properties.Srcs)