device_iot_config: Add mock and fix stack btm test

This patch adds missing device_iot_config API mock.
Regression was triggered due to some StackBtmTests
enabling all the init flags. The device_iot_config
module was not initialized for tests causing the
tests to break.

Bug: 266133076
Tag: #feature
Test: atest bluetooth_test_gd
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 lines

Change-Id: Ib34e44a8673a2234050bbca0e10b92a017f57b82