Only display options to save and cancel edits to the displayed APN if the APN is editable.

Test: mp settingsg and look at the UI. This class does not yet have
tests.

Current design:
 - https://screenshot.googleplex.com/BgrLPsz3ebxBnMa
 - https://screenshot.googleplex.com/AeRQe6Rh8SxjjMw

Updated design: https://screenshot.googleplex.com/3bhmWwMjhEXh2Eu

Change-Id: Ic9794ca14534adde92a641560a00b1bd876d52cb