enc stat: Add Encoding Statistics keys to Codec2 layer

Codec2 layer does not know about the feature of 'EncodingStatitics',
and only knows about each individual statistics, i.e. average_qp,
picture_type.

Bug: 205793764
Bug: 203696003
Test: cts
Change-Id: Ief16ee5499fa11f788dfb73590ba4ac756376dd3