nblog: split NBLog.cpp into multiple files

NBLog.cpp was deleted and split up into Entry.cpp, Merger.cpp,
Reader.cpp, Timeline.cpp, and Writer.cpp. NBLog.h was split up into
Entry.h, Events.h, Merger.h, Reader.h, Timeline.h, and Writer.h. NBLog.h
now includes all of these new headers except Merger.h, which is managed
exclusively by MediaLogService.

Other changes:
- changed NBLog from a class to a namespace
- changed NBLog::thread_info_t.type to enum and replaced magic numbers
- fixed some fragile build dependencies in libaudioclient and medialog

No other changes were made, and all functionalities that existed prior
to this commit remain the same.

Test: build, dumpsys media.log
Bug: 68148948
Change-Id: I1ce6dcb7b30d004f90584edfad7103a89f7bab0c