- unpin participant (if the local one was pinned it would remain)
- close any dialog (except if authentication is pending)

Co-authored-by: DimaG <dgeorgiev06@gmail.com>

Analytics is erroring when unpinning because the logged
event sends null for the objectId. The objectId should
be the id of the person getting unpinned.

This feature is intended for spot. Spot can have an
HDMI -> usb adapter hooked up to it. In that case,
attempting to screenshare should use that adapter
as a screensharing source. Jitsi-Meet should pass
a configured screenshare source into lib-jitsi-meet
so it can be used as a source.

The class name is similar to the one used for the mute button and is used by the tests to locate and click the button.

Provide a default and builtin default implementation which finishes the
Activity, same as before.

What this PR removes is the ability to provide a custom default handler because
applications can already take this decision when calling `onBackPressed`. In
addition, make `onBackPressed` return `void` because it's virtually impossible
for it to return `false` (that would mean that there is no
`ReactInstanceManager`, which means there is no app to begin with).

In addition, remove the use of `BackAndroid` since `BackHandler` contains an iOS
shim now.

We no longer need it since Rect 16 takes care of passing the context around.
It's also deprecated: https://atlaskit.atlassian.com/packages/core/layer-manager

Instead of handling the side effect of navigating to another
url from within componentWillReceiveProps, try to match the
same logic instead in componentDidUpdate.

Touch feedback manifests in some ugly black border bleeding out of the thumbnail
itself. Since we already provide feedback (be that by adding the blue border in
case of pinning, or showing the menu in case of long press) the perception is
the same, without the graphical glitch.

- Reset some state on the singletons conference
  and VideoLayout.
- Add a way for LocalVideo to clean itself up
  by sharing logic with the other SmallVideos.
- Add clearing of chat messages so they don't
  linger.
- Remove some UI event listeners.

componentWillMount is a deprecated lifecycle method;
componentDidMount should be used to kick off things
like ajax. In the case of the _App hierarchy, a promise
chain is used to perform initialization, and it is
first started in the constructor by initializing
storage. However, by the time storage is initialized,
resolving the first promise, _App has already mounted.
So, move it all to the componentDidMount lifecycle.

The package now requires using a ModalTransition component
to handle animations. The existing DialogContainer component
has been split into native and web implementations to support
this change.

Apple rejected our app on account of requiring Dropbox not being acceptable. Oh
well! Disable it until we find a way around it. Sigh.

It's possible for the YouTube api to return zero broadcasts
or broadcasts without any streams--streams are what are
associated with stream keys. In this case, instead of showing
an empty selector or no selector, show a message with a link
to where the stream key can be obtained.

Provide a client-side notice if the YouTube live stream key
looks like it might be in the wrong format. Normally the
stream key looks like 4 groups of 4 numbers and letters,
each separated by a dash. The warning does not block submission
in case YouTube changes their stream key format.

Updating react-native-fast-image brings a couple of interesting changes:

- onLoad is not called for cached images (reported and ignored upstream)
- load progress not working if component not displayed (on Android)

In order to fix this, a combination of 2 approaches was used:

- onLoadEnd / onError are used to detect if the image is loaded
- off-screen rendering is used on Android to get progress events

While implementing the above, yours truly noticed the complexity was increasing
way too much, so some extra refactoring was also performed:

- componentWillReceiveProps is dropped
- an auxiliary component (AvatarContent) is used for the actual content of the
  Avatar, with the former passing the key prop to the latter

Using the key prop ensures AvatarContent will be recreated if the URI changes,
which is not a bad idea anyway, since the new image needs to be downloaded.