[Android] Fix resource leak when JitsiMeetView is destroyed

In iOS this is automagically done in the view destructor, bunt we don't have
that luxury in Java we have to do it manually.

The new disponse() method MUST be called when the Activity holding the view is
going to be destroyed, typically in the onDestroy() handler.

android/README.md

     protected void onDestroy() {
         super.onDestroy();
 
+        view.dispose();
+        view = null;
+
         JitsiMeetView.onHostDestroy(this);
     }
 
 The `JitsiMeetView` class is the core of Jitsi Meet SDK. It's designed to
 display a Jitsi Meet conference (or a welcome page).
 
+#### dispose()
+
+Releases all resources associated with this view. This method MUST be called
+when the Activity holding this view is going to be destroyed, usually in the
+`onDestroy()` method.
+
 #### getListener()
 
 Returns the `JitsiMeetViewListener` instance attached to the view.

android/sdk/src/main/java/org/jitsi/meet/sdk/JitsiMeetActivity.java

     protected void onDestroy() {
         super.onDestroy();
 
+        if (view != null) {
+            view.dispose();
+            view = null;
+        }
+
         JitsiMeetView.onHostDestroy(this);
     }
 

android/sdk/src/main/java/org/jitsi/meet/sdk/JitsiMeetView.java

         views.add(this);
     }
 
+    /**
+     * Releases the React resources (specifically the {@link ReactRootView})
+     * associated with this view.
+     *
+     * This method MUST be called when the Activity holding this view is destroyed, typically in the
+     * {@code onDestroy} method.
+     */
+    public void dispose() {
+        if (reactRootView != null) {
+            removeView(reactRootView);
+            reactRootView.unmountReactApplication();
+            reactRootView = null;
+        }
+    }
+
     /**
      * Gets the {@link JitsiMeetViewListener} set on this {@code JitsiMeetView}.
      *
 
         // TODO: ReactRootView#setAppProperties is only available on React
         // Native 0.45, so destroy the current root view and create a new one.
-        if (reactRootView != null) {
-            removeView(reactRootView);
-            reactRootView = null;
-        }
+        dispose();
 
         reactRootView = new ReactRootView(getContext());
         reactRootView.startReactApplication(reactInstanceManager, "App", props);