Adding cached value for current user for per-SysUI process components.

- Also ensuring that we don't do additional work for non-current users when
  the task stack changes.

Bug: 38390446
Test: Launch Recents in multiple users, ensure that it works and does not
      make a binder call to get the current user id

Change-Id: If075675edc3874d809bad30d0aa564a8e59771d3