unicodejs: Make tool executable, fix path

* The file has a hash bang, but wasn't executable so it still
  required running like `$ python ./tools/unicodejs-properties.py`

* When called from a parent or sibling directory, it wrote to
  an unexpected file path. Fixed by expanding relative to the
  script's directory instead.

Change-Id: Ia369b98cdeb87845af3e6777e9bcf6bf04ec4592