Clean up AppLabelProxy

Clean up usages of AppLabelProxy in Telecom -- we had 3 or 4 instances
of the same logic lying around, and this change centralizes all of them
and adds more logging.

Change is in preparation for and to debug the null app name bug from
background call screening.

Bug: 150214111
Test: existing unit tests, manual
Change-Id: I0e8e757c9a62eaa2db11b7dc6e8b369ffb799322