Files app: Hide Download folder inside Play files.

"Play files/Download" is an alias to Chrome OS's Downloads volume.
It is convenient in Android file picker, but can be confusing in
Chrome OS Files app.
This CL hides the 'Play files/Download" only in Chrome OS Files app.

Bug: 848661
Test: Manually tested on arc-enabled Kevin.

Change-Id: I6c9bed8ff7f72b7c1fe6b2abe1c6194f8eeb3b6d
Reviewed-on: https://chromium-review.googlesource.com/1166922
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581834}

ui/file_manager/file_manager/foreground/js/directory_contents.js

@@ -429,6 +429,7 @@ function FileFilter(metadataModel) {
    */
   this.lastHostedFilesDisabled_ = null;
 
+  this.hideAndroidDownload();
   chrome.fileManagerPrivate.onPreferencesChanged.addListener(
       this.onPreferencesChanged_.bind(this));
   this.onPreferencesChanged_();
@@ -534,6 +535,23 @@ FileFilter.prototype.isAllAndroidFoldersVisible = function() {
   return !('android_hidden' in this.filters_);
 };
 
+/**
+ * Sets up a filter to hide /Download directory in 'Play files' volume.
+ *
+ * "Play files/Download" is an alias to Chrome OS's Downloads volume. It is
+ * convenient in Android file picker, but can be confusing in Chrome OS Files
+ * app. This function adds a filter to hide the Android's /Download.
+ */
+FileFilter.prototype.hideAndroidDownload = function() {
+  this.addFilter('android_download', entry => {
+    if (entry.filesystem && entry.filesystem.name === 'android_files' &&
+        entry.fullPath === '/Download') {
+      return false;
+    }
+    return true;
+  });
+};
+
 /**
  * @param {Entry} entry File entry.
  * @return {boolean} True if the file should be shown, false otherwise.