Add playback and capture callbacks for echo canceller algo

[nanang]

Currently, PJMEDIA EC module will invoke EC algo via single operation callback only, such as cancel_echo(play_frame, rec_frame, out_frame), while some EC algo may not provide such API model, instead two operation APIs, i.e: playback() and capture(), are the only provided, so this two APIs model should be supported.

Note that with two APIs model, we assume that PJMEDIA EC module should leave the latency buffering to be handled by the EC algo.

[nanang]

In 4622:

Close #1705: Added playback and capture callbacks for echo canceller algo. Note that this changeset also modified current Speex AEC algo behaviors:

• applied this two APIs model for Speex AEC algo
• enabled Speex AGC preprocessing