Fix to avoid generation id mismatch after data setup completion

During data setup connection process generation id will be incremented in the apncontext on which the connection is about to setup. But if a data connection is scheduled for retry and on the same apncontext if a new connection is attempted and rejected the generation id count is left incremented which leads to mismatch in the generation id currently held in apncontext and the generation id on which the connection was successful. So decrement the generation id of the apncontext when connection attempt is rejected.

Change-Id: Iba107945b78438053848f16d40a18b11f66dfd64
CRs-Fixed: 2623859