recorder: refactor and simplify

1. Adds constness to streams in MediaRecorder; the latter shouldn't change
stream parameters.
2. addStream is called from recordData to simplify workflow; users need
only check that MediaRecorder is recording.
3. Fixes regression where multiple records couldn't happen in the same
call.
4. Fixes AudioSender not sending starting timestamp to help mix audio.
5. MediaStream::name is now used as key for the streams_ map, so it must
be constant, filter names are no longer hardcoded in the recorder.
6. fromPeer parameter is determined by checking if MediaStream::name
contains the string "remote".
7. Adds copy constructor to MediaStream.
8. Renames incrementStreams to incrementExpectedStreams to better reflect
what it does.
9. Removes keyframe triggers when starting a video record; we are
recording raw frames, so it doesn't matter if they're keyframes or not.
10. MediaRecorder supports more than 2 audio streams, but not video.
11. Move preview to very bottom right corner.
12. Fix potential Unchecked Return Value to NULL Pointer Dereference in
recordData() (CWE-690).

Gitlab: #39

Change-Id: Id2fae4e9bb2072994c065e4843cc3ad832f89efc