Floss: Run queued gatt operations when the previous one completes

bta_gattc implements a queue p_q_cmd_queue so incoming commands might
be queued in case there are other running operations. However, the
queue is only checked if some operation fails to be run.

This CL also checks the queue after some operation completes.

Bug: 271827876
Test: run CTSV LE Insecure Client Test
Change-Id: Ia7eeb7b1ad89d06cebd7663a9d839b36d0ff53a0