Support opening speaker only in pjsua/pjsua2

[nanang]

Currently pjsua will always open both speaker and microphone. Some applications, such as Push-To-Talk, may have microphone off most of the time and prefer to open only the speaker.

While working on pjsua2 sound device, there seem to be a related issue, calling setPlaybackDev() & setCaptureDev() consecutively may not work as expected, i.e: for changing both playback and record devices, perhaps due to asynchronous dev open/close operations, this is experienced on Linux platforms. So, perhaps we should consider to add an API to set both at once, e.g: setPlaybackAndCaptureDev().

[riza]

In 5273:

Re #1908: Implement new API to open speaker only in pjsua/pjsua2.