Use constant names instead of literals

Use constant names instead of literals when writing annotations to
increase readability.

FrameworkStatsLog.java: https://paste.googleplex.com/4789132691767296
statslog.h: https://paste.googleplex.com/4986214782337024
statslog.cpp: https://paste.googleplex.com/4856851575341056

Bug: 151786433
Test: m statslog-framework-java-gen
Test: m libstatslog
Change-Id: I47111fe02319bebf41f429642fb675f7a856710b