Add uint16 support for tf.decode_raw (#12719)

* Add uint16 support for tf.decode_raw

This fix tries to address the request raised in 10124 where
uint16 support for tf.decode_raw is needed. tf.decode_raw
already support half, float32, float64, int8, int16, int32, int64,
uint8. And uint16 was not supported.

This fix adds uint16 support for tf.decode_raw.

This fix fixes 10124.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Fix test failure caused by uint16 support of decode_raw and add unit tests.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>