Use a partition packaging spec filter for android_filesystem

This revives https://r.android.com/3308803. The use case for this filter
is
1. To remove the transitive shared library dependencies of `system_ext`
   binaries that are intended to be installed in `system`. Without this
   filter, we get two copies of libc.so, libz.so, ... on device
2. To filter out dexpreopt files of apps. Even if an app is installed in
   `/system_ext`, its dexpreopt files will be installed in
   `system_other`. Kati allows this by allowing ctx.InstallFile to
   specify the path relative to device root.

Diff in autogenerated `system_ext` partition for aosp cf before and
after this CL https://diff.googleplex.com/#key=6ekb09PtreNh

Test: go test ./filesystem
Bug: 372487849
Change-Id: Ie52723be2469d3b210ee1ebb62997d378d44788b