Revert "Change symlink_forest to use relative symlinks."

Revert submission 2673616-relativesymlinks-fix

Reason for revert: this breaks tests verified with go/abtd https://android-build.googleplex.com/builds/abtd/run/L36000000963001181

Bug: 300122962

Reverted changes: /q/submissionid:2673616-relativesymlinks-fix

Change-Id: I5a97c4fbe4df5727c0604a07137093d0f00c7776