Refactors the previous "[RN] Cache avatars and provide a default in
case load fails" for the purposes of simplification but also modifies
its functionality at the same time. For example:

- Always displays the default avatar immediately which may be seen if
  the remote avatar needs to be downloaded.
- Does not use random colors.
- Uses a default avatar image which is not transparent and ugly but at
  least it's the same image that's used on Web. I've started talks to
  have images/avatar2.png replaced with a transparent and beautiful
  so that will land later on and we'll see the automagic colors in all
  their glory then.

Avatars are cached to the filesystem and loaded from there when requested again.
The cache is cleaned after a conference ends and on application startup
(defensive move).

In addition, implement a fully local avatar system, which is used as a fallback
when loading a remote avatar fails. It can also be forced using a prop.

The fully local avatars use a user icon as a mask and apply a background color
qhich is picked by hashing the URI passed to the avatar. If no URI is passed a
random color is chosen.

A grace period of 1 second is also implemented so a default local avatar will be
rendered if an Avatar component is mounted but has no URI. If a URI is specified
later on, it will be loaded and displayed. In case loading the remote avatar
fails, the locally generated one will be used.

The files styles.js are used (pretty much) on React Native only and each
of them exports 1 name. Export it as default to cut down on source code.

When entering audio-only mode, VideoBridge is instructed to stop sending
remote videos. However, if the instruction fails because DataChannels do
not work, for example, then the app continues to display the remote
videos. Even though they're received in the case of such a failure, no
videos are to be displayed in audio-only mode.

Introduce certain React Components which may be used to write
cross-platform source code such as Audio like Web's audio, Container
like Web's div, Text like Web's p, etc.

Also move (native) avatar style to film-strip styles, since  that's where it
applies. This is analogous to how the large-view avatar is styled.

This reverts commit 739298c782.

They better represent if a participant has video available or not. There are
cases when even a participant in the last N set would not have video because it
disconnected momentarily, for example.