Fix transaction interference in v9

[schmidt-sebastian]

This fixes a bug in listener unregistration, which exists in both the user facing unsubscribe API as well as runTransaction() (since it uses listeners internally).

When we unregister a listener, we go through all existing listeners to find the one that matches. We do this by comparing the callbacks for equality.

In v8, we wrap all listeners, and hence, we have to unwrap the listener to get back to the original user function before checking for equality. Our listener looks as such:

const listener = () => { userCallback(); /*...*/ )
listener.userCallback = userCallback;

In v9, we have:

const listener = userCallback(); 

Unfortunately, the comparison function considered all v9 listeners the same since it either compared listener (which only matches for the exact listener) or listener.userCallback (which matched every listener since the property was never set).

Fixes #5195

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  Package "firebase-compat-typings-test" must depend on the current version of "firebase-exp": "9.0.0-beta.7" vs "file:../../packages-exp/firebase-exp"
  Package "firebase-compat-typings-test" must depend on the current version of "firebase-exp": "9.0.0-beta.7" vs "file:../../packages-exp/firebase-exp"
  Package "firebase-compat-typings-test" must depend on the current version of "firebase-exp": "9.0.0-beta.7" vs "file:../../packages-exp/firebase-exp"
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[egilmorez]

When there are no changes to comments you don't really need any TW review, but I do approve, thanks! :)