Create statsToken in ImeRequestedChangedListener

This adds REASON_IME_REQUESTED_CHANGED_LISTENER and starts tracking IME
requests without statsToken from the IME requested changed listener.
While this does lead to some requests which are soon cancelled (e.g. at
PHASE_SERVER_SHOULD_HIDE), it allows observing requests that would
otherwise be picked up just before they are finished, in
InputMethodService showSoftInput/hideSoftInput.

This is only an improvement, but not a final solution for tracking new
IME requests from the refactorInsetsController changes.

Flag: EXEMPT logging
Bug: 353463205
Test: show and hide IME in various scenarios, observe logs
Change-Id: Ie71cc4ef1effed7c7927060937f23742aa13ecb1