L2E: Make SimpleEncode take vp9 level as an input

L2E: Make SimpleEncode take vp9 level as an input

Level conformance is standadized in vp9.
If a specific target level is set, the vp9 encoder is required to
produce conformant bitstream with limit on frame size, rate,
min alt-ref distance, etc.

This change makes the SimpleEncode environment take the target level
as an input.

To make existing tests pass, we set the level to 0.

Change-Id: Ia35224f75c2fe50338b5b86a50c84355f5daf6fd