Add PreferenceChangeListener to ApnSettings

The PreferenceChangeListener was removed from aosp/779634, when the
selected apn is changed, the result is not saved to the database.

In this CL, we add the PreferenceChangeListener back.

Bug: 121229775
Test: build & manual test
Change-Id: Ief1a2e05c072e5eff8060731b044c19878d346be
1 file changed