[Chromecast] Increase min buffer time for ALSA mixer

Our timing for delayed tasks is only accurate to about 15ms or so; the
min buffer time should be bigger than that so that underrun prevention
actually works.

Bug: 
Change-Id: I40cf658ea4d131cc8c6eaeeae5d3323575d0d798
Reviewed-on: https://chromium-review.googlesource.com/691302Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505308}

chromecast/media/cma/backend/alsa/stream_mixer_alsa.cc

@@ -85,7 +85,7 @@ const int kDefaultCheckCloseTimeoutMs = 2000;
 
 // The minimum amount of data that we allow in the ALSA buffer before starting
 // to skip inputs with no available data.
-const int kMinBufferedDataMs = 8;
+const int kMinBufferedDataMs = 20;
 
 // A list of supported sample rates.
 // TODO(jyw): move this up into chromecast/public for 1) documentation and