Prevent deadlock between the Tx and Rx threads in HCI layer

Problem: Deadlock between HCI layer's Tx and Rx threads.

Cause: In HCI layer, both Tx and Rx tried to acquire
commands_pending_response_mutex simultaneously.
Since there may be a handshake between Tx and Rx
in the BT Transport layer, so it caused a deadlock between
HCI layer's Tx and Rx threads.

Fix: Fixed it by decreasing the scope of this mutex in Tx thread

Test: Simulated the deadlock sitaution and ensured RX thread is returned when TX is busy in binder.

Bug: 68305277
Change-Id: I0a2434ec24918b039a6993d57e68ac7495e31cbe