Fix issue where voicemail number is not saved properly

When VoicemailSettingsActivity resumes from background, all the
preferences are once removed and recreated. This causes the issue
where voicemail number input by user on old preference is not saved.
To fix the issue, keep the reference to old preference and don't
override it in onResume().

Test: manual - checked if voicemail number is saved
Bug: 29736548
Change-Id: I4437a8827eb8338ccd78826316b684d7fac532c7
1 file changed