Nuplayer logging empty records to media.metrics

Changes to ensure that nuplayer always reported a particular set
of metrics through android.media.MediaPlayer.getMetrics() changed
the shape of what we should have considered as an empty record.
This caused some noise in the reported media metrics logs.

Also removed a no-longer-wanted ALOGD() diagnostic.

Bug: 36658856
Test: read 'dumpsys media.metrics' output