CRAS: dev_stream - track read offset per device.

Similar to how output devices work, track each stream's read offset
into the device.  This will allow streams that need SRC to read less
than the maximum amount of frames.

BUG=chromium:421941
TEST=play and record from multiple devices.

Change-Id: I1a5445bee41e17196557e9cce2795f97ed66f1b8
Signed-off-by: Dylan Reid <dgreid@chromium.org>