ui: LoginActivity: set alert dialog for device spoof

Currently, AppAuth doesn't have any implementation on sending the device
credentials and google takes the device name from within the Browser info
to be shown at Google Account->Security->Devices which is basically getting
spoofed due to some security reasons.

So just put up an alert dialog before the sign in to make user aware of
the fact and also for avoiding unnecessary panics on their side.

Issue was reported here https://gitlab.e.foundation/e/apps/AccountManager/-/issues/51
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