title: FirebaseAuth creating custom tokens
body: <p>When you create a custom token using the below method, is a unique token generated every time even if the uid is the same? Or will it always generate the same token for the same uid in an application?</p>
  <pre><code>FirebaseAuth.getInstance().createCustomToken(uid)
  </code></pre>
  <p>Also, how long is the generated token valid for? For the lifetime of the application until revoked?</p>