libprocessgroup: fix reset of file_v2_name

ProfileAttribute::Reset does not reset file_v2_name, fix that. Also
provide ProfileAttribute::file_name() to consolidate the code.

Bug: 292636609
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I5b33ca47b4fa5cabf582c8804bd13f72f6e58411