Fix apn can't be updated when pressing back key

APN can't be updated because onkey() method is not invoked when user presses
the virtual back key. The ApnEditor was changed from Activity to Fragment
in Android N. A fragment can't receive onKey() event without calling
setFocusableInTouchMode(true).

Test: manual - updated apn setting, then pressed back
Bug: 32815214
Change-Id: I7824e84bd617d2057a61f4198fa7fd50f123bc8e