Build ramdisk's build.prop with soong

Following the same pattern as existing soong build.props, but with
minor tweaks for ramdisk such as making the partition "bootimage".

Bug: 378146476
Test: Diff'd out/target/product/vsoc_x86_64/ramdisk/system/etc/ramdisk/build.prop before and after this cl
Change-Id: I7ab2621d78c28deb73628bc455e040247a3c8031