[HostStubGen] Code cleanup

- Reduce unnecessary output filter layering: a lot of these filters are
  orthogonal to each other, so there is no need to do "layering" as
  these layers do not interact with each other. Merge them all into
  InMemoryOutputFilter.
- Consolidate the construction of the ClassVisitor layers into
  HostStubGenClassProcessor: previously the logic to build these layers
  are scattered in multiple places. Put them all into the same method to
  make it easier to understand and update these ClassVisitors.

Bug: 292141694
Flag: EXEMPT host side change only
Test: f/b/r/scripts/run-ravenwood-tests.sh
Change-Id: Ibec23a1282c5013541cfe2beb977d25d81bf4bb5