Add support installing to root of filesystem image

If the partition of a spec is "root", that spec will be installed to
root instead. Normally that spec will be from prebuilt_root module with
install_in_root property.

Bug: 351258461
Test: USE_SOONG_DEFINED_SYSTEM_IMAGE=true m && cvd start
Change-Id: Iaaa9c2fb8a81fe0ba4710c641e1b65c5b71ad4a4