Move definition of create-vendor-odm-symlink within BUILDING_VENDOR_IMAGE

When we build the system image separately from the vendor image, we need the
/vendor/odm link (if any) to be created in the vendor build, not the system
build. This change moves the create-vendor-odm-symlink function outside the
BUILDING_SYSTEM_IMAGE gaurd, and inside the BUILDING_VENDOR_IMAGE gaurd.

Bug: 136516335
Test: Build vendor image, verify that /vendor/odm -> /odm
Change-Id: I6edf5d6d1fa0b2f632f67113f69a49b43f719941