logd: Add Tag statistics

(cherry pick from commit 344bff43)

- Optional class of statistics for events Tags
- export tagToName from LogBuffer (located in main.cp to address
  https://android-review.googlesource.com/#/c/110204)
- Can not handle dropped because getTag() can not work,
  will need to be fixed if we start filtering.

Bug: 19608965
Change-Id: I7b90607ca588bf37bab71f19b1570a290e772776