Allow setting buffer length for HotwordAudioStreamManager

A HotwordDetectionService can specify a buffer length for each
HotwordAudioStream in the HotwordDetectedResult by adding an int in the
HotwordAudioStream's metadata PersistableBundle, with the key
HotwordAudioStream.KEY_AUDIO_STREAM_COPY_BUFFER_LENGTH_BYTES.

If the value is not specified or is an invalid value, the default buffer
length is 2,560 bytes.

Bug: 257509629
Test: Manually with test app, verified with logs
Test: atest CtsVoiceInteractionTestCases
Change-Id: I28c7f21bdf7c00b62c9cc6c29153d79ce9d85286