Deprecate null IME token rule in IMM#setInputMethod()

With my previous CL [1], InputMethodManagerService#setInputMethod() is
now guaranteed to be called only from IInputMethodManager and
IInputMethodPrivilegedOperations as 'adb shell ime set' no longer
directly calls this method (with null IME token).

With this CL, IInputMethodManager#setInputMethod(), which has been
kept just for null IME token rule, is finally gone. This is achieved
by letting InputMethodManager#setInputMethod() directly update
DEFAULT_INPUT_METHOD (and SELECTED_INPUT_METHOD_SUBTYPE) secure
settings if a priviledged component still relies on this undocumented
null IME token rule.

 [1]: I6fd47b5cc1e7da7222774df20247a2c69a70f45b
      db25df71

Fix: 114488811
Test: atest CtsInputMethodServiceHostTestCases
Change-Id: I42dd0325b01c527009bf85566ca8ba0766b2294e