Clean up vendor image handling

Standardize symlinking /system/vendor -> /vendor for aosp_* devices,
since some /vendor binaries still use /system/vendor/... paths.

Support using a prebuilt vendor image and including it into all the
normal packaging steps.

Bug: 28987532
Change-Id: I27040e8a8d1df0777e16cd1e3c3a9f1b28695e96