Fix a race condition when creating rfcomm servers

It is a race condition issue between 2 rfcomm server connection with the
same channel number. I changed the SCN allocated policy. BT stack
always allocate new SCN even the SCN has been released.

Test: manual test
Bug: 243974739
Change-Id: I0e535ff846f6745a9d6225ddb231f9ec3f8f7a19