adb: identical symlink, do not update

Inefficient to always update the symlinks to overlayfs, can
also lead to

    failed to copy 'xxx' to 'yyy': remote symlink failed: File exists

on older variants of overlayfs that do not effectively mark them
deleted.

Test: manual
Bug: 109821005
Change-Id: If1286d76f45ce14087cdb515fe8d2fed198fd9d8