Add apex_test for mixed builds

Test: go test soong tests
Change-Id: I33327e1fe7d6a4bbf32890d9fbd5453c28a9c5b1
diff --git a/apex/apex.go b/apex/apex.go
index a3872d3..cb5a5e9 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -1893,10 +1893,13 @@
 	a.outputFile = a.outputApexFile
 	a.setCompression(ctx)
 
+	// TODO(b/257829940): These are used by the apex_keys_text singleton; would probably be a clearer
+	// interface if these were set in a provider rather than the module itself
 	a.publicKeyFile = android.PathForBazelOut(ctx, outputs.BundleKeyInfo[0])
 	a.privateKeyFile = android.PathForBazelOut(ctx, outputs.BundleKeyInfo[1])
 	a.containerCertificateFile = android.PathForBazelOut(ctx, outputs.ContainerKeyInfo[0])
 	a.containerPrivateKeyFile = android.PathForBazelOut(ctx, outputs.ContainerKeyInfo[1])
+
 	apexType := a.properties.ApexType
 	switch apexType {
 	case imageApex: