Telephony: Add support for tune away and priority sub

Add constants for tune away and priority subscription

Change-Id: I790cfdcd9a1dee946502349084a4d8138d2144ec
CRs-fixed: 464483
(cherry picked from commit ebaedd5e9dc33feda0fb0a0c47e204c4efa41873)
(cherry picked from commit 1c534171e946a46c15bfadb5f0732a6b32cc5ec7)