build.prop for ODM partition is under ./etc subdir

build.prop for the ODM partition should be placed under ./etc directory
to support GSI. For detailed reasons, see
I0733c277baa67c549bb45599abb70aba13fbdbcf

The path was changed to /odm/build.prop during the recent refactoring
around build.prop filems and this change fixes the mistake.

Bug: 132128501

Test: check if odm/etc/build.prop is created
Change-Id: Icc2c564250aa08786f32294a839cc81e86b7d368