Remove "ask each time" SIP call setting. (1/2)

1. Removed setting from Settings.System class.
2. Added settings database migration step to change "SIP_ASK_ME_EACH_TIME"
to "SIP_ADDRESS_ONLY" as the "ask me each time" option no longer makes
sense given the new phone accounts settings.

Bug: 17321422
Change-Id: I3df1be4fcda44f2097c49af44508ac1fce72a24b