LogPager: separate getter from query filter

LogPager: separate getter from query filter

The method LogPager::getFilterParams combines data aggregation and query
modification which is problematic: it starts with "get" and is public
but has a side-effect. Instead, make a dedicated private method for
building the query and remove the side-effect.

The new private method is now dedicated to building the query, and so
it can also make sure whether selection by log id is requested. If it's
the case, it won't filter the log types out (fix for T220834).

Also remove redundant comment: it's irrelevant to require callers to
call a private method.

Bug: T220834
Change-Id: I0fb6e6ee30ac626192cdf4ef920a731c03b2732f