Add function to initialize MediaFormat audio & video

[sauwming]

Currently, the object MediaFormatAudio/Video will be uninitalised upon creation (i.e. type == PJMEDIA_TYPE_NONE).

To initialise it, user needs to explicitly set its parent's type to PJMEDIA_TYPE_AUDIO/VIDEO, otherwise the object will still considered to be uninitialised.

For example:

VideoPreview prev(videoDevIdx);
VideoPreviewOpParam param = VideoPreviewOpParam();
param.format.id = PJMEDIA_FORMAT_I420;
param.format.width = 1080;
param.format.height = 1920;
param.format.fpsNum = 15;
param.format.fpsDenum = 1;
/* Despite all the above initialisations, param.format.type is still PJMEDIA_TYPE_NONE and hence, will just be ignored. */
prev.start(param);

[nanangizz]

Notes:

• The default media type in the PR description is PJMEDIA_TYPE_UNKNOWN while it should be PJMEDIA_TYPE_NONE instead (they have different values)?
• Better also quickly scan our sample apps for possible bug caused by this behavior.