Moving /odm/build.prop to /odm/etc/buid.prop

In device root directory, we have the following symlinks:
  - /odm/app -> /vendor/odm/app
  - /odm/bin -> /vendor/odm/bin
  - /odm/etc -> /vendor/odm/etc
  ...

This allows the Generic System Image (GSI) to be used on both devices:
  1) Has a physical odm partition, where those symlink will be hidden
     when /odm is used as the mount point
  2) Has no physical odm partition and fallback to /vendor/odm/.

We can't just have the symlink /odm -> /vendor/odm, because the former
devices won't have /vendor/odm directory, which leads to mount failure
when the mount point /odm is resolved to /vendor/odm.

The existing /vendor/odm/build.prop won't be loaded in the latter
devices, because there is no symlink:
    - /odm/build.prop -> /vendor/odm/build.prop.

Note that init blocks reading through direct symlinks (O_NOFOLLOW) so
the above symlink won't work either. This CL moves the odm build.prop
to /odm/etc/build.prop for init to load it (symlinks in earlier
components of the path will still be followed by O_NOFOLLOW).

Bug: 132128501
Test: boot a device and checks /odm/etc/build.prop is loaded
Test: make dist with an odm.img, checks $OUT/odm/etc/build.prop is loaded
Change-Id: I6f88763db755c9ec6068bfdd9cee81c19d72e9d7