[Files app] A11y announce file selection changes

Change file list and file grid view to announce file selection changes
when triggered by user input (keyboard, mouse and tap). The following
actions are announced on selection changes, via aria-live:

  1. Selecting a single file entry.
  2. Selecting a range of file entires.
  3. Adding a file entry to the current selection.
  4. Removing a file entry from the current selection.
  5. Selecting all file entries.
  6. Deselecting all files entries.

Add i18n translation support for these messages.

Change ToolbarController, FileGrid and FileTable to receive an
{A11yAnnounce} implementation to be able to send aria-live messages.

Add getItemLabel to FileGrid, FileTable and FileTableList to be able to
retrieve item's label based on the selection index to be able to add
the item name/label to the aria message.

Change test util RemoteCall.waitAndClickElement to accept modifier keys
so we can send Ctrl and Shift modifiers for testing multiple file entry
selection in FilesApp integration test.

While here, change == and != to === and !==. Also call the constructor
|super| instead of "cr.ui.table.TableList.prototype.mergeItems.call".

NOTE: Add tests only for Keyboard and Mouse for now, because the touch
emulation event isn't triggering file selections yet.

Test: browser_tests --gtest_filter="*KeyboardSelectionA11y* : *MouseSelectionA11y*"
Bug: 888636
Change-Id: I1bfdcf3987a7bcf8d29bfce6468c3b109f5fb239
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1638299
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666567}