Log PS WWAN RAT from registration info

Log the actual cellular RAT instead of the combined RAT.
Note that on IWLAN legacy mode device, modem might still
reports IWLAN as the data RAT.

Test: Manual + unit test
Bug: 131927497
Bug: 144389598
Bug: 145759808

Merged-In: Ifdab565fc23938b3a5780268abb761bdc1da82c1
Change-Id: Ifdab565fc23938b3a5780268abb761bdc1da82c1