logcat: lastLogTime only two most recent files

Scaling issues if -f logrotation is specified and there are a lot of
log files to process in lastLogTime, focus on only the last two log
files. This will cover the situation where we have recently rotated
the logs and the last file is missing or empty.

This also alters monotonic processing as well as it used to focus on
only the last file; the danger being doubled now if the pair covers
more than one reboot when filling in missing content with logcat -L.
Problem was always there with one log file, but now we handle if
rotation happened recently. A fair tradeoff for KISS.

Bug: 30630936
Bug: 30612424
Bug: 28788401
Change-Id: I1c13b94e88f5edc08ecef52d407e296874ca5807