Update WebChromeClient FileChooserParams for File System Access API

* Unhide WebChromeClient.FileChooserParams.MODE_OPEN_FOLDER
* Add new getPermissionMode() function and
  PERMISSION_MODE_{READ,READ_WRITE} constants. This indicates the
  intended mode for opening a file or dir and can be used as a hint to
  use OPEN_DOCUMENT Intent rather than GET_CONTENT (read-only).

Bug: 40101963
Bug: 364980165
Test: manual
Flag: android.webkit.file_system_access
API-Coverage-Bug: 364980165

Change-Id: I6f8cd5323c547656a3de82e2d7f8ff0b9eb7fc3a