fix(SVideoWriter): remove hard-coded codec and only offer mp4 as only extension available in ui

Description

• Limits extension selection to only .mp4 for now.
• Limits FOURCC codec to avc1. This codec is also linked to the .mp4 extension.

Closes #33 (closed)

How to test it?

• Launch ExVideoRecorder sample
• Select a device (webcam)
• Click on the "video recording" button (the second red button)
• Type in a file name followed by an extension or you can just type a filename with no extension. In the latter case, the selected extension in the UI will be set.

⚠ for now, only .mp4 with avc1 codec is allowed.