Remove IInputMethod.attachToken(IBinder) from greylist

Apps can never directly use IInputMethod.attachToken(IBinder) in a
meaningful way, because

 1. The real binder object is always instantiated in the IME process
    and its proxies will never be exposed to any other process except
    for the system server process.  Even if there was actually a real
    use of this method (via reflection), then the caller must have
    been the same IME process.

 2. However, InputMethodImpl.attachToken(IBinder) does nothing if it
    gets called twice for security reasons, because the fact it can be
    called only once is guaranteed in the protocol between IME and
    InputMethodManagerService. This means that even if an IME somehow
    called this Binder method via reflection, it must be either no-op
    (if it's a secondary call) or make itself useless (if it's the
    first call).

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

Bug: 112722706
Test: compile
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: Ie5e5b3a5f379c42cf2d5f681b556c60aeccee9f9