Fix preferred apn not selected when back from edit

When back from edit page, preferred apn and apn list will both refresh,
if apn list refresh happens later, it will clear the preferred apn.

Saved the latest preferred apn into mPreferredApnKey to fix.

Bug: 257316932
Test: manual
Change-Id: I62f9cbd5847b7edb834c815cdecc089e0764a8c7