Switch the FS type of generic_system_image to erofs

The kati built system image used by cuttlefish uses erofs with
compression. Since generic_system_image (soong-built) is now used in
aosp_cf_* products, this CL switches the fs type to `erofs` from `ext4`
(the default).

system.img size
- before 1.1 GB
- after 759 MB

Test: presubmits
Change-Id: Id5b0b8901aa90788cce975f3393d37e6c4844d47