AudioService: BtHelper: fix LE profile callback registration

Add missing unregistration of the LE profile proxy callback when
the profile is disconnected.

Also protect from multiple on profile connected calls received
with the same profile proxy.

Bug: 333834324
Test: make
Flag: EXEMPT trivial bug fix
Change-Id: I3489b313cc2786c1cd1e560640663123b673d8c6