[firebase-auth] currentUser property keeps the user object after signing out

[canmertc]

firebase().auth().currentUser property stores the User object on a successful sign-in as expected. But it keeps storing the user object after signing out. currentUser getter returns the first signed-in user even after signing in to another user account.

⚠️ Because of firebase().auth().currentUser.getIdToken() returns the first user's token; It causes the 2nd user to use the first user's id token when hitting API endpoints.

Steps to reproduce

1. Sign in (Account A)

console.log(firebase().auth().currentUser) // User (Account A)
console.log(firebase().auth().native.getCurrentUser()) // [Android] Native user (Account A)
console.log(firebase().auth().native.currentUser) // [IOS] Native user (Account A)

2. Sign out (Account A)

console.log(firebase().auth().currentUser) // Still has the user data of Account A [BUG]
console.log(firebase().auth().native.getCurrentUser()) // [Android] null
console.log(firebase().auth().native.currentUser) // [IOS] null

3. Sign in (Account B)

console.log(firebase().auth().currentUser) // Still has the user data of Account A [BUG]
console.log(firebase().auth().native.getCurrentUser()) // [Android] Native user (Account B)
console.log(firebase().auth().native.currentUser) // [IOS] Native user (Account B)