NBLog: add retroactive dump and standard printing format

The retroactive dump outputs historical snapshots of data that
correspond to important NBLog Events at the time they were read.

The standard printing format added is as follows: for each line, the
first column is the NBLog Event as a string, and the following columns
correspond to each field of the struct, or the type, that the NBLog
Event maps to. All columns are separated by commas.

Other changes:
Added option to Reader::getSnapshot() for flushing the buffer. This
makes it possible to always read the full buffer contents when doing a
dump of the raw log contents.

Test: dumpsys media.log with -r and --retro options
Bug: 68148948
Change-Id: I4bbbf6e2f9a3a86fae976986ac35a7eef79520de