[auth][android] signOut return undefined

[kefahB]

Hello @NathanWalker and @triniwiz

The signOut on android return undefined instead of boolean! the workaround is fixed by adding async await but this will force us to change the return type to Promise! and if we did we should doing same for IOS as well.

What do you think ?

https://github.com/NativeScript/firebase/blob/889726640381cb4a0cdbe855f658ff3245a0eb3d/packages/firebase-auth/index.android.ts#L1398