Bug 919215 - Start the AudioStream on creation when in low-latency mode, and let it underrun. r=roc

The BufferedAudioStream buffers the data it gets through the Write() calls and
what is consumed by the callback. This means that if the audio producer starts
Write()ing data right after Start()ing the stream, data will accumulate in this
buffer and won't be consumed. Eventually, the buffer will be of a certain size
before it begins to be consumed by the callback, and this means an
umcompressible latency (because the data will be written at more or less the
same rate as it is produced).

This patch start the BufferedAudioStream right away when it is created, dropping
the silent AudioSegment until it finds real data (and padding with silence is
then done at the beginning). The stream will underrun, but the callback will
synthetize silence, avoiding overbuffering in the BufferedAudioStream. This
ensures minimal latency cause by the buffering.

Note that the clock will still advance, so this will not change the behavior of
content that has leading silence.