Transmit pending L2CAP fixed channel packets before closing

SMP closes L2CAP channel immediately after sending its last packet.
L2CAP may not have transmitted all the SMP packets by the time it
receives remove request. As a result, it may end up flushing the pending
SMP packets. This causes the SMP pairing to fail on the remote device
due to timeout.
The change ensures that pending packages in L2CAP fixed channels are
transmitted when the channel is closed locally.

Test: mmm packages/modules/Bluetooth
Test: Manual | Pair with LE-only devices. Pair with dual mode devices
over BR/EDR
Flag: com.android.bluetooth.flags.transmit_smp_packets_before_release
Bug: 314417619
Bug: 357106628

Change-Id: I7e474f14ce5e8195e2f67e3ac5af8e2c8c2cb10f