Phone: fix my phone number not saving

- The if condition was wrong, so it only saves when the saved
  number is equal to the new number!
- Also set persist to false to disable automatic saving as
  "button_pn_key"

Change-Id: I6f4950e8a36e591c380c0cdb1785a63aa98da8d4
merge changes from CyanogenMod 10.2 into local branch