Passing the default apex version to the apexer

Test: presubmit
Bug: 231691643
Change-Id: I900a6acde9f212bf785b0be35f422416c8b62792
diff --git a/apex/builder.go b/apex/builder.go
index 9119363..73e9b73 100644
--- a/apex/builder.go
+++ b/apex/builder.go
@@ -650,6 +650,8 @@
 			optFlags = append(optFlags, "--manifest_json "+a.manifestJsonOut.String())
 		}
 
+		optFlags = append(optFlags, "--apex_version "+defaultManifestVersion)
+
 		optFlags = append(optFlags, "--payload_fs_type "+a.payloadFsType.string())
 
 		ctx.Build(pctx, android.BuildParams{