Retain "voicemail:" URI for startOutgoingCall and add scheme Constants.

We want the "voicemail:" URI to be passed to InCallUI, so do
not convert to "tel" before startOutgoingCall. Also factor out
"SCHEME_*" constants to a separate file.

Bug: 17363355
Change-Id: I6ee42bb5262d4de8121082681ccd4f2f06b74b95