findbugs: Clean up ST warnings

Removed all-out ST category suppression and replaced it with narrow,
targeted suppressions, so that future instance methods updating static
variables will be caught by FindBugs.

Change-Id: I158d22e2c7e5f1ccedb6782b3288473f33e47a3f
Signed-off-by: Allon Mureinik <amureini@redhat.com>