Reduce unnecessary overhead of SystemUiContext

- Do not report change when initializing DisplayContent
  (DisplayContent#isReady() is false).
  This reduces boot time by dozen of milliseconds
  (RootWindowContainer#setWindowManager).
- Only create SystemUiContext if needed. Most of processes don't
  use it. This reduces many registrations (e.g. according to the
  number of processes, it maybe from ~60 to ~10), which involve
  binder/listener creation/invocation, especially the extra cost
  of dispatching configuration to the context which no one uses.
- Store token as IWindowToken to avoid object creation every time
  by asInterface.

Bug: 207620458
Test: atest InputMethodMenuControllerTest WindowContextControllerTest
Change-Id: I867e9f81116796c42048195406d74feccf4772d3