app crash on browsing folder with a lot of photos: `RejectedExecutionException` in `DisplayUtils.setThumbnail`

### Cause of error
```java
Exception in thread "main" java.util.concurrent.RejectedExecutionException: Task android.os.AsyncTask$3@fb5b3d8 rejected from java.util.concurrent.ThreadPoolExecutor@e399331[Running, pool size = 17, active threads = 17, queued tasks = 128, completed tasks = 26]
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2078)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:843)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1389)
at android.os.AsyncTask.executeOnExecutor(Unknown Source:34)
at com.owncloud.android.utils.DisplayUtils.setThumbnail(DisplayUtils.java:952)
at com.owncloud.android.ui.adapter.OCFileListDelegate.bindGridViewHolder(OCFileListDelegate.kt:92)
at com.owncloud.android.ui.adapter.OCFileListAdapter.onBindViewHolder(OCFileListAdapter.java:366)
at androidx.recyclerview.widget.RecyclerView$Adapter.onBindViewHolder(RecyclerView.java:7254)
at androidx.recyclerview.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:7337)
at androidx.recyclerview.widget.RecyclerView$Recycler.tryBindViewHolderByDeadline(RecyclerView.java:6194)
at androidx.recyclerview.widget.RecyclerView$Recycler.tryGetViewHolderForPositionByDeadline(RecyclerView.java:6460)
at androidx.recyclerview.widget.GapWorker.prefetchPositionWithDeadline(GapWorker.java:288)
at androidx.recyclerview.widget.GapWorker.flushTaskWithDeadline(GapWorker.java:345)
at androidx.recyclerview.widget.GapWorker.flushTasksWithDeadline(GapWorker.java:361)
at androidx.recyclerview.widget.GapWorker.prefetch(GapWorker.java:368)
at androidx.recyclerview.widget.GapWorker.run(GapWorker.java:399)
at android.os.Handler.handleCallback(Unknown Source:2)
at android.os.Handler.dispatchMessage(Unknown Source:4)
at android.os.Looper.loop(Unknown Source:152)
at android.app.ActivityThread.main(Unknown Source:65)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(Unknown Source:11)
at com.android.internal.os.ZygoteInit.main(Unknown Source:338)

```

### App information
* ID: `com.nextcloud.client`
* Version: `30200190`
* Build flavor: `gplay`

### Device information
* Brand: `Xiaomi`
* Device: `clover`
* Model: `MI PAD 4 PLUS`
* Id: `OPM1.171019.019`
* Product: `clover`

### Firmware
* SDK: `27`
* Release: `8.1.0`
* Incremental: `V10.3.1.0.ODJCNXM`