ref: move all prop type declaration to flow

For the most part the changes are taking the "static propTypes" declaration off
of components and declaring them as Flow types. Sometimes to support flow some
method signatures had to be added. There are some exceptions in which more had
to be done to tame the beast:
- AbstractVideoTrack: put in additional truthy checks for videoTrack.
- Video: add truthy checks for the _videoElement ref.
- shouldRenderVideoTrack function: Some component could pass null for the
  videoTrack argument and Flow wanted that called out explicitly.
- DisplayName: Add a truthy check for the input ref before acting on it.
- NumbersList: Move array checks inline for Flow to comprehend array methods
  could be called. Add type checks in the Object.entries loop as the value is
  assumed to be a mixed type by Flow.
- AbstractToolbarButton: add additional truthy check for passed in type.

react/features/mobile/network-activity/components/NetworkActivityIndicator.js

@@ -1,31 +1,28 @@
 /* @flow */
 
-import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 import { connect } from 'react-redux';
 
 import { LoadingIndicator } from '../../../base/react';
 
+/**
+ * The type of the React {@code Component} props of
+ * {@link NetworkActivityIndicator}.
+ */
+type Props = {
+
+    /**
+     * Indicates whether there is network activity i.e. ongoing network
+     * requests.
+     */
+    _networkActivity: boolean
+};
+
 /**
  * The React {@code Component} which renders a progress indicator when there
  * are ongoing network requests.
  */
-class NetworkActivityIndicator extends Component<*> {
-    /**
-     * {@code NetworkActivityIndicator} React {@code Component}'s prop types.
-     *
-     * @static
-     */
-    static propTypes = {
-        /**
-         * Indicates whether there is network activity i.e. ongoing network
-         * requests.
-         *
-         * @private
-         */
-        _networkActivity: PropTypes.bool
-    };
-
+class NetworkActivityIndicator extends Component<Props> {
     /**
      * Implements React's {@link Component#render()}.
      *