[hoststubgen] Reusable TextFileFilterPolicyParser

- Make TextFileFilterPolicyParser reusable.
- Tweak the "dump" test to address b/388562869

Flag: EXEMPT host test change only
Bug: 388607679
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh -s

Change-Id: Ib5ab0e4c5070f46704fdc14400fec0f06c430990