Fix a potential ODR vialation

Evaluation of (BTM_BLE_CONFORMANCE_TESTING == TRUE)
is dependent on whether bt_target.h is included or not.

When bt_target.h is not included, neither BTM_BLE_CONFORMANCE_TESTING
nor TRUE are defined, (BTM_BLE_CONFORMANCE_TESTING == TRUE)
evaluates to true; When bt_target.h is included
(BTM_BLE_CONFORMANCE_TESTING == TRUE) evaluates to false.

This resulted in a violation of ODR (one definition rule) and
caused net_test_stack_btm to fail in aosp/2817294.

This change uses ifdef BTM_BLE_CONFORMANCE_TESTING.

Bug: 309029578
Test: atest net_test_stack_btm
Change-Id: Ic03b9808657d08ceef770e9dc1b031a0af46029d