Move build.prop to ./etc subdir for some partitions

build.prop files under system_ext, product, and odm partitions are moved
to the ./etc subdirectory. This is to be in-line with the established
practice of having all configuration files under ./etc subdir.
build.prop doesn't need to be an exception.

However, system/build.prop and vendor/build.prop were not moved because
there are a lot of apps and tools that depend on the path.

Bug: 158735147
Test: device has build.prop under ./etc and is bootable
Change-Id: I37c79f7337680f92406d9e196e5762bb8e7da5d8