Split chrome/browser/ash/fileapi:fileapi GN target into :impl

This is an precursor step to modularize //c/b/file_system_access,
which itself is an precursor step to modularize //c/b/ui/file_system_access.

In practice, it avoids the following circular dependency:

  ERROR Dependency cycle:
  //chrome/browser/ash/arc:arc ->
  //chrome/browser/ash/app_restore:app_restore ->
  //chrome/browser/ash/arc/session:session ->
  //chrome/browser/ash/arc/accessibility:accessibility ->
  //chrome/browser/ash/arc/input_method_manager:input_method_manager ->
  //chrome/browser/ash/input_method:input_method ->
  //chrome/browser/ash/file_manager:file_manager ->
  //chrome/browser/ash/file_manager/virtual_tasks:virtual_tasks ->
  //chrome/browser/ash/fileapi:fileapi ->
  //chrome/browser/file_system_access:file_system_access ->
  //chrome/browser/permissions:permissions ->
  //chrome/browser/ash/app_mode/web_app:web_app ->
  //chrome/browser/ash/login/ui:ui ->
  //chrome/browser/ash/arc:arc

... because :fileapi includes headers from the upcoming
:file_system_access target.

Bug: 366272539

Change-Id: I9ae121a5989a7bf7d03912308f3da6f72d1e02de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5857876
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1355114}