fix(ngMessages): prevent race condition with ngAnimate

If `ngMessage` tried to add a message back in that was about to be removed
after an animation, the NgMessageController got confused and tried to detach
the newly added message, when the pending node was destroyed.

This change applies a unique `attachId` to the message object and its DOM
node when it is attached. This is then checked when a DOM node is being
destroyed to prevent unwanted calls to `detach`.

Closes #12856
Closes #12903