Copy abidiffs into /abidiffs on abi breakages.

Bug: 64267858

Test: create reference abi dump for libjpeg locally. Change return type
      of libjpeg_std_err and build with :
      BOARD_VNDK_VERSION=current mm -j64 showcommands dist
      Build fails and abidiff report gets copied into out/dist/abidiffs.

Change-Id: I7c8ecfac95361e731009e5913bd3a7bb323a9597

android/variable.go

@@ -189,6 +189,7 @@ type productVariables struct {
 	Override_rs_driver *string `json:",omitempty"`
 
 	DeviceKernelHeaders []string `json:",omitempty"`
+	DistDir             *string  `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {