BTM: Do not start authentication twice

Originally, this code used an alarm to delay authentication requests.
When an incoming authentication request arrived, the alarm was
checked first. That check was removed with the alarm, so this CL
adds a check to see if the device is authenticating before starting
the process.

Bug: 270161453
Test: atest pts-bot
Change-Id: I0cd19a3070656c3882323d1fa87caed1ee330e1e