[Chromecast] Widevine decrypt supports both clear and secure output

For OEMs supporting OEMCrypto_DecryptCENC, we should be able to get both
clear and secure output, as far as it enforces the license policy.

On devices with audio mixer, cast will use clear audio buffer and
secured video buffer.

CMA backend can use DecryptContext::Decrypt to decrypt data into a
secured buffer.

CMA pipeline can use DecryptContextImpl::DecryptAsync to decrypt data
into a clear buffer.

BUG=internal b/72180017
TEST=encrypted stream playback, cast_cdm_widevine_test

Change-Id: I78a77cd7bfbdfb421cf5c6a6a651a01397c55dc0
Reviewed-on: https://chromium-review.googlesource.com/985031


Reviewed-by: Sergey Volk <servolk@chromium.org>
Commit-Queue: Sergey Volk <servolk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546884}