Add a "post-processing" step for text policies

Text policy is a special case, because it can override any inner-level
policies, while being targeted mostly for development. This means that
sometimes we need additional logic applied during policy resolution.

- Merge PackageFilter into InMemoryOutputFilter to reduce unnecessary
  filter layering
- Add a unified post-processing method to apply common logic across all
  policy types
- Always use the inner policy if the text policy does not actually
  "override" the inner policy behavior to make the policy reason more
  accurate.

Bug: 292141694
Flag: EXEMPT host side change only
Test: f/b/r/scripts/run-ravenwood-tests.sh
Change-Id: Ib8f7c5023e205ee5e616523079eff6d20b841590