Allow ExternalStorageProvider to access all Android/ dirs

In Scoped Storage world, access to "Android/data" style directories are
hidden for privacy reasons. Files Activity by DocsUI calls
ExternalStorageProvider in managed mode. Allow ExternalStorageProvider
to show these hidden files when in manage mode.

Bug: 150366834
Test: Android/data and Android/obb dirs are not visible in DocsUI Picker
      view (triggered from StorageTestApp or Gmail)
Test: Android/data and Android/obb dirs are accessible from DocsUI Files
      view (triggered from Settings). Android/data shows package dirs.
      Manually adb push video to /obb is accessible.

Change-Id: I2423d8fa695c68a56d630353317b98ff3bb7ad34