logd: add internal prdebug function

Usage: android::prdebug(const char *fmt, ...) __printflike(1, 2);

Will add logd prefix tag, log as debug priority in kernel logs,
and will suffix a newline if one is not supplied. To be used to
aid debugging of the logger only.

Change-Id: I217326ef34dc4eb0ea076bacb7a7a8b564c931c3