Introduce accessibility large pointer icons.

This patch does the following things:
- add new graphic assets of large icons for accessibility needs
  (imported from ChromeOS data)
- add the logic to choose the set of normal icons or accessibility icons
- make InputManagerService to observe the settings change, so that
  it can reload the new resources

This patch misses non-1x graphic though, because ChromeOS doesn't
have such data.

Bug: 25778857
Change-Id: Ia5f95d47f50b3f6eea555c3af8069bc6bae0b400