`series.dt.tz_localize()` on Categorical operates on categories, not values
#### Code Sample, a copy-pastable example if possible

```python
datetimes = pd.Series(['2019-01-01', '2019-01-01', '2019-01-02'], dtype='datetime64[ns]')
categorical = datetimes.astype('category')
categorical.dt.tz_localize(None)
```

Produces:

```
0   2019-01-01
1   2019-01-02
dtype: datetime64[ns]
```

#### Problem description

`.dt.tz_localize()` is operating on `categorical.cat.categories`. It should be operating on `categorical.astype('datetime64[ns]').values`. This is just plain wrong.

#### Expected Output

According to [Categorical docs](https://pandas.pydata.org/pandas-docs/stable/user_guide/categorical.html#string-and-datetime-accessors), "The returned Series (or DataFrame) is of the same type as if you used the .str.<method> / .dt.<method> on a Series of that type (and not of type category!).". So I think the expected value to be:

```
>>> datetimes.dt.tz_localize(None)
0   2019-01-01
1   2019-01-01
2   2019-01-02
dtype: datetime64[ns]
```

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.2.8-200.fc30.x86_64
machine          : x86_64
processor        :
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.0
numpy            : 1.17.0
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.0.2
setuptools       : 40.8.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.3.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : 2.8.3 (dt dec pq3 ext lo64)
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : 4.7.1
bottleneck       : None
fastparquet      : 0.3.2
gcsfs            : None
lxml.etree       : 4.3.0
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
</details>
