Remove IInputMethodManager.{add,remove}Client() from greylist

Apps can never directly IInputMethodManager.{add,remove}Client() in a
meaningful way, because

 1. These Binder methods are implemented in InputMethodManagerService
    (IMMS) but unnecessarily exposed to application processes.  The
    only valid caller of these methods is WindowManagerService.
    Actually with my recent change [1], any incoming call of these
    methods from the application processes will always be rejected.

 2. The purpose of these callbacks is maintaining a per-process
    callback (IInputMethodClient) from each application process to
    IMMS.  Since WindowManagerService is responsible for dealing with
    this step, there is no reason for app developers to call these
    accidentally exposed hidden Binder methods.  If there was an
    application that was trying to call these methods via reflection,
    it must be trying to bypass caller verification of some other IME
    APIs by registering a fake IInputMethodClient callback.

 3. However, even if an application succeeded to register a fake
    IInputMethodClient to IMMS, IMMS still has additional verification
    IWindowManager.inputMethodClientHasFocus(IInputMethodClient),
    which cannot be spoofed with such a fake connection.

Therefore having these entries hiddenapi-light-greylist.txt must be
either a mistake or no-op.

 [1]: Ib9b588d11bd4017e431e3d494863987dd67384fc
      6efd55e7

Bug: 112670859
Bug: 112722706
Test: compile
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I62aa27ceb97446059ce80b00a3369c81b1c70e23