VP9 denoising enabled by noise_sensitivity param

As in VP8.

Currently, this parameter is set with the VP8E_SET_NOISE_SENSITIVITY flag.
The flag was not renamed so that we don't break the interface for webrtc. This
should probably be changed at some point in the future.

Change-Id: Ic73fcb0dde9d1d019e9d042050b617333ac65472