Fix some AppCompacted proto nits post google3 review.

Clarify some comments and tweak the names of some fields. Binary
structure of the proto remains intact.

Bug: 119988524
Bug: 120600464

Test: verified that statsd_testdrive still works with this atom
Change-Id: I90d8eb522fc23b2ff604bc1e69e4068dc2c4b73a