[Chromecast] Adjust cast_audio_output_stream mixer output buffer sizes.

Currently audio output streams inside MixerInputConnection have a fixed
maximum buffer size of 8192 frames. This means that the overall delay
changes depending on sample rate (with lower sample rates accumulating
substantial delay, 512ms at 16000Hz).

The current 8192 frame buffer is equivalent to 170ms of audio at
48000Hz, which is unnecessarily high. Set non-communications streams to
70ms and communications streams to 50ms.

confirmed both with logging and listening test that the latency is
substantially reduced.

Bug: internal 168254375
Test: On device. Ran sample app at both 48000 and 16000 sample rates and
Change-Id: I109df7dd10e3067f9cb4f9117c31e96cbdef581b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461218


Commit-Queue: Adam Whiteside <awhiteside@google.com>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815452}