Add a stem value to handcrafted soong system images

These get copied to the staging directory as system.img, but adding a
stem value ensures that the filename is system.img is also in
intermediates. This is useful for secondary packaging tasks such as
target_files.zip generation

Test: presubmits
Change-Id: I5e1d61c5c41d221bf1be6a061990e985a0044ce5