replace char * with std::string

change some statsd parameters that were formerly bare C strings to
use std::string; statsd changed behavior from Q (NULL was tolerated) to
R (NULL causes crash within statsd).

Bug: 157887206
Bug: 155363739
Test: playback of protected content
Change-Id: Id3f65c43212f20d1b042a445753d57432c94a259