Fixed incorrect merge service state

The existing logic for merging service state when device is on
IWLAN does not work properly on AP-assisted mode devices. Fixed
by copying the IWLAN preferred flag into IMS service state so
it can get the correct data RAT for AP-assisted mode devices.

Bug: 134706530
Test: WFC manual tests
Change-Id: Ic18934adb870a0114c913855cdb68e0ef33caa79