Improvements to echo cancellation framework

[bennylp]

Modify the echo cancellation framework as follow to make it easier to integrate with new echo cancellation backends, and to generally improve it. These changes should be backward compatible.

• Support for multi-channel echo cancellation:
  • Add new API to specify channel count
  • Modify EC backends to support multi-channel EC (latest Speex AEC supports this)
• Change the semantic of options flag when creating the EC, to allow application to choose which EC backend to use when there are multiple backend algorithm.
• Move the latency compensation functionality from Speex EC (echo_speex.c) to the generic EC framework (echo_common.c) so that it can be reused by other backends.
• Since the common EC framework now does the buffering, remove the ec_playback() and ec_capture() internal API from the backends. The backends now only need to supply ec_cancel() API, which is simpler, and this opens support for more backends.
• Add new reset API.
• Modify the aectest.c to work with WAV files instead of live recording. This would make it easier to test.

[bennylp]

Done in r2198. Also in this changeset:

• roll-back Speex mdf.c from rc1 to beta3 version as the latest one does not work.

[bennylp]

Note:

• with the previous commit, Speex AEC currently does not support multi-channel EC. This will be updated by ticket #589