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 value of inviteURL is derived from locationURL by removing the hash
and query/search params in order to make it fit for display and/or
public purposes. The Jitsi Meet SDK consumers do not fall into that
category and our intention is to provide them with the URL they used
with JitsiMeetView.openURL(URL) anyway.

Also rewrites to remove repetition. I'm not saying the new source code
is better really but at least I got to examine it and comment on some of
its weaknesses.

The current implementation doesn't use the API and Transport modules. This is
due to the fact that they are too tied to APP at the moment, which is web only.

Once API is refactored and moved into the Redux store this will be adjusted,
though it's unlikely that the lowest level React Native module (ExternalAPI)
changes drastically.

This commit also introduces a stopgap limitation of only allowing a single
instance for JitsiMeetView objects on both Android and iOS. React Native doesn't
really play well with having multiple instances of the same modules on the same
bridge, since they behave a bit like singletons. Even if we were to use multiple
bridges, some features depend on system-level global state, such as the
AVAudioSession mode or Android's immersive mode. Further attempts will be made
at lifting this limitation in the future, though.

When a dialog is opened on Android, full-screen mode is exited but we (the app)
know nothing about this. Make sure we re-enter full-screen mode once a dialog is
closed, if the conditions to be in such mode are still met.

When the audio-only mode is enabled, turn on the proximity sensor to dim the
screen and disable touch controls when there is an object nearby.

The behavior can be triggered with the toggleAudioOnly action, which is
currently fired with a button.

The following aspects of the conference will change when in audio only mode:

- local video is muted
- last N is set to 0 (effectively muting remote video)
- full-screen mode is exited
- audio mode is set to "audio chat" (default output is the earpiece)
- the wake lock is disengaged

One aspect not handled in this patch is disabling the video mute button while in
audio only mode. The user should not be able to turn back video on in that case.

This is in preparation for an upcoming "audio only mode" feature. Setting last N
will also be required for it, so this patch factors out the action and makes it
public so other modules can reuse it.

In addition, if the value is set to undefined the configured default value (or
-1 if absent) is picked.