L2CAP Socket: Keep track of last allocated socket ID

* Keep track of last allocated socket ID in L2CAP socket stack
* Use last_sock_id + 1 as new IDs when allocating new socket blocks
* The de-dupe and overflow detection mechanism in btsock_l2cap_alloc_l()
  would handle the uint32_t overflow and duplicate ID cases

Test: CtsBluetoothTestCases
Bug: 144148429
Change-Id: Ieb9791ffa34eef919a32e4aff6f4b514859c69c0