Change Android MediaStore path for MyFiles

Changing it to /storage/MyFiles-read, which provides a readable view of
the MyFiles from app with WRITE_EXTERNAL_STORAGE permission.

BUG=b:162791183
TEST=Photos can see file changes.

Change-Id: Ibdb218f426339bf14fc8a571581a86c7849153ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359595Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Satoshi Niwa <niwa@chromium.org>
Commit-Queue: Risan <risan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800017}

chrome/browser/chromeos/arc/file_system_watcher/arc_file_system_watcher_service.cc

@@ -50,13 +50,15 @@ constexpr base::FilePath::CharType kAndroidDownloadDir[] =
     FILE_PATH_LITERAL("/storage/emulated/0/Download");
 
 // The MyFiles path inside ARC container. This will be the path that is used in
-// MediaScanner.scanFile request.
+// MediaScanner.scanFile request. MediaScanner scans MyFiles under
+// /storage/MyFiles-read instead of /storage/MyFiles because non-system apps has
+// no access to /storage/MyFiles.
 constexpr base::FilePath::CharType kAndroidMyFilesDir[] =
-    FILE_PATH_LITERAL("/storage/MyFiles");
+    FILE_PATH_LITERAL("/storage/MyFiles-read");
 
 // The path for Downloads under MyFiles inside ARC container.
 constexpr base::FilePath::CharType kAndroidMyFilesDownloadsDir[] =
-    FILE_PATH_LITERAL("/storage/MyFiles/Downloads/");
+    FILE_PATH_LITERAL("/storage/MyFiles-read/Downloads/");
 
 // The removable media path inside ARC container. This will be the path that
 // is used in MediaScanner.scanFile request.