Make symlink creation in `tools/buildbot-setup-device.sh` more robust.

Check for existing symlinks before creating them.

Test: Run `art/tools/buildbot-setup-device.sh` twice and check for errors
Bug: 160559664
Change-Id: I777e9677c47e9c1003f1a1e18f9a8d1fd85cc7bc
1 file changed