Clarify nullness about "token" param of IMMS#calledWithValidTokenLocked()

Now that InputMethodManagerService#calledWithValidTokenLocked() is
called to verify callers of InputMethodPrivilegedOperationsImpl, it is
now clear that its "token" parameter is guaranteed to be non-null.

It's time to close TODO comments added 2 years ago [1].

This CL is still a mechanical refactoring.  There should be no
behavior change.

 [1]: I3fcdeb919bb2f2940da9ff45e17ac00baa1253f4
      d0332835

Fix: 34851776
Bug: 34886274
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: Idc514a0ce714bc857966e5f1afd452d4aeda1f5f