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
Merged-In: I798f1d968821e7d1b0a97b9b034c1ef0726f8242
(cherry picked from commit c384c930)