feat: add methods to check muti-mic support and stats collecting

For audio previewing, some browsers cannot have multiple local
audio streams at once. For audio output previewing, local
stats need to be collected to detect changes in volume level.

Pull Request #435

JitsiMediaDevices.js

 import * as JitsiMediaDevicesEvents from './JitsiMediaDevicesEvents';
 import * as MediaType from './service/RTC/MediaType';
 import RTC from './modules/RTC/RTC';
+import RTCBrowserType from './modules/RTC/RTCBrowserType';
 import RTCEvents from './service/RTC/RTCEvents';
 import Statistics from './modules/statistics/statistics';
 
         }
     },
 
+    /**
+     * Returns true if it is possible to be simultaneously capturing audio
+     * from more than one device.
+     *
+     * @returns {boolean}
+     */
+    isMultipleAudioInputSupported() {
+        return !RTCBrowserType.isFirefox();
+    },
+
     /**
      * Returns currently used audio output device id, 'default' stands
      * for default device

JitsiMeetJS.js

 import JitsiTrackError from './JitsiTrackError';
 import * as JitsiTrackErrors from './JitsiTrackErrors';
 import * as JitsiTrackEvents from './JitsiTrackEvents';
+import LocalStatsCollector from './modules/statistics/LocalStatsCollector';
 import Logger from 'jitsi-meet-logger';
 import * as MediaType from './service/RTC/MediaType';
 import Resolutions from './service/RTC/Resolutions';
         return this.mediaDevices.isDeviceChangeAvailable(deviceType);
     },
 
+
+    /**
+     * Checks if the current environment supports having multiple audio
+     * input devices in use simultaneously.
+     *
+     * @returns {boolean} True if multiple audio input devices can be used.
+     */
+    isMultipleAudioInputSupported() {
+        return this.mediaDevices.isMultipleAudioInputSupported();
+    },
+
+    /**
+     * Checks if local tracks can collect stats and collection is enabled.
+     *
+     * @param {boolean} True if stats are being collected for local tracks.
+     */
+    isCollectingLocalStats() {
+        return Statistics.audioLevelsEnabled
+            && LocalStatsCollector.isLocalStatsSupported();
+    },
+
     /**
      * Executes callback with list of media devices connected.
      * @param {function} callback

modules/RTC/JitsiLocalTrack.js

  */
 JitsiLocalTrack.prototype._setRealDeviceIdFromDeviceList = function(devices) {
     const track = this.getTrack();
+
+    // FIXME for temasys video track, label refers to id not the actual device
     const device
         = devices.find(
             d => d.kind === `${track.kind}input` && d.label === track.label);

modules/statistics/LocalStatsCollector.js

  * Starts the collecting the statistics.
  */
 LocalStatsCollector.prototype.start = function() {
-    if (!context
-        || RTCBrowserType.isTemasysPluginUsed()) {
+    if (!LocalStatsCollector.isLocalStatsSupported()) {
         return;
     }
     context.resume();
     }
 };
 
+/**
+ * Checks if the environment has the necessary conditions to support
+ * collecting stats from local streams.
+ *
+ * @returns {boolean}
+ */
+LocalStatsCollector.isLocalStatsSupported = function() {
+    return Boolean(context && !RTCBrowserType.isTemasysPluginUsed());
+};
+
 module.exports = LocalStatsCollector;