Don't update preference list when there's no Activity for it.

Phone account setting will crash due to NullPointerException if
PhoneAccountSettingsFragment not recycled immediately after dialer's
main activity started.

Bug: 197199688
Test: Manually start phone account setting activity and open dialer
in lockscreen and make sure there's no crash.

Change-Id: Ic72ba80fd699f19f55aab00dafd3510fdb966720
1 file changed