Use LRU to cache the Avatar shown in push notifications (#7124)

* iOS switch from file cache to memory cache and use last_picture_update to update the avatar if needed

* Android switch from file cache to memory cache and use last_picture_update to update the avatar if needed, split function to multiple files and catch potential exceptions

android/app/src/main/java/com/mattermost/rnbeta/NotificationsModule.java

@@ -1,7 +1,6 @@
 package com.mattermost.rnbeta;
 
 import android.app.Notification;
-import android.app.NotificationManager;
 import android.content.Context;
 import android.os.Bundle;
 import android.service.notification.StatusBarNotification;