wifi(implementation): Simplify getFirstActiveWlanIfaceName

Get rid of the unnecessary looping inside the method which was causing a
bunch of unnecessary selinux denial logs.

Bug: 130384921
Test: ./data/android.hardware.wifi@1.0-service-tests
Test: Manually verified the property name after wifi on/off & softap
on/off.

Change-Id: Ia9f61749b2017234851d67b027deb685cb8b539e