Rename FormSummaryAggregator to SummaryAggregator

There is nothing form-specific in FormSummaryAggregator, and we need
exactly the same thing for senses as well, so rename it to a more
generic SummaryAggregator. Comments referencing forms or form elements
are shuffled around and rephrased to become more generic and, hopefully,
more understandable as well.

Bug: T198196
Change-Id: Ie5026d584e1b0d14432a967dfcbce3072a3abc4f