[clang-tidy] Disable bugprone-narrowing-conversions for mediametrics

Bug: http://b/181927912
Bug: http://b/182410845

This warning re-appeared:

/mnt/disks/build-disk/src/googleplex-android/master/frameworks/av/services/mediametrics/statsd_drm.cpp:139:33:
error: narrowing conversion from 'size_t' (aka 'unsigned long') to signed type 'std::__wrap_iter<unsigned char *>::difference_type' (aka'long') is implementation-defined [bugprone-narrowing-conversions,-warnings-as-errors]
        buf.erase(buf.begin() + size, buf.end());

Let's disable this and fix the warning after the compiler update lands.

Test: m statsd_drm.tidy
Change-Id: Ibabf5b9caaa77d201543308724c8444d98c5dde1