add api to get sections of procstats

Add api to collect committed procstats data.
Use highwatermark from the user to filter which files to include. This
way multiple users in the system can use this api.

Also allow ProcessStats.java to dump optional ProcessStatsSectionProto
so that other users can use it.

Test: will add cts test
Change-Id: If37c0fd25814460a1c8d5996d24d138295ff6421