logd: add Chattiest LOG_TAG statistics

Report global LOG_TAG usage.

Switch NULL to nullptr and use const more accurately.

Test: gTest liblog-unit-tests, logd-unit-tests & logcat-unit-tests
Test: manual: inspect logcat -S results around 'Chattiest TAGs'
Test: logcat -b all -c ; logcat -b all -S ; then confirm clear
Bug: 37254265
Change-Id: I3696c0d8da3ba24f99f670aafba1e45f8cb3ab14