Adds image-specific selinux_fc props to allow different file_contexts.

This enables mixed builds to use the file_contexts.bin from the system
build when regenerating images that come from system target files, and
similarly for file_contexts.bin from the other build when regenerating
images from other target files.

In monolithic (non-mixed) builds all image-specific selinux_fc props
point to the same file_contexts.

Bug: 132108151
Test: Built and booted mixed build devices.
Change-Id: Id51ed6d96ea6337879f1ab21d47c93c67bc25312