FileStream can be used to output audio playback buffer to a file. It
receives a memory mapped file and makes it act as an audio buffer.
BUG=b:233542268
TEST=emerge crosvm
Change-Id: I2b013fe0422db8475145537adf9c36492f52ae26
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4306259
Reviewed-by: Chih-Yang Hsia <paulhsia@chromium.org>
Reviewed-by: Dennis Kempin <denniskempin@google.com>