L2CAP SignallingManager: Store command_just_sent

Instead of storing it in pending_commands_ *after* a command is sent, we
use a separate variable command_just_sent_ to store it, to avoid state
corruption

Test: bluetooth_test_gd and run_cert.sh
Bug: 145622504
Change-Id: I5992b3545c04ec2bc36fcc43ae84b81a6b26eb8f