[HostStubGen] clean-up remapper support

Using "Pair" was too ad-hoc. Let's blend the feature into OutputFilter.

This will allow more flexible remapping in the future too.

Flag: EXEMPT host side test change only
Bug: 358155729

Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh
Change-Id: Idf954e4536ef1332571ae22fd0217ef760427b87