Index * Series returns Index
Holdover from #19042 (see discussion in #19044), the example below applies to regular Index objects too.  Easy fix (tag as good-first-PR?), just need to add a check [here](https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/base.py#L4022) that returns `NotImplemented` if `isinstance(other, ABCSeries)`.  Updating the appropriate tests is the labor-intensive part.

```
tdi = pd.TimedeltaIndex(['0days', '1day', '2days', '3days', '4days'])
ser = Series([0, 1, 2, 3, 4], dtype=np.int64)
expected = Series(['0days', '1day', '4days', '9days', '16days'],
                          dtype='timedelta64[ns]')

result = tdi * ser
tm.assert_series_equal(result, expected)   # --> fails

>>> result
TimedeltaIndex(['0 days', '1 days', '4 days', '9 days', '16 days'], dtype='timedelta64[ns]', freq=None)
```
  