Add telecom to priorityOnlyDndExemptPackages

telecom plays dialer's ringtone, so we allow them to play sounds when
DND is on in priority mode. We trust that telecom will abide by the DND
rules and exceptions (ie: checking for starred contacts)

Test: manual
Bug: 163525045
Change-Id: I798f1d968821e7d1b0a97b9b034c1ef0726f8242