Allow IMM to forward API calls to IMS

This is one further step towards deprecating 8 IME APIs that were
accidentally defined InputMethodManager (IMM) instead of
InputMethodService (IMS).

With this CL, API calls to those 8 deprecated ones in IMM will be
forwarded to IMS so that we can completely remove corresponding IPC
methods from IInputMethodManager.aidl.  This guarantees that processes
that have no InputMethodService running there become unable to access
IPC methods behind such IME APIs that are intended to be used only
from IMEs.

One tricky thing is that the following 4 public APIs have been allowed
to processes that have WRITE_SECURE_SETTINGS permission, even if such
a process does not have active InputMethodService.

 * InputMethodManager.setInputMethod
 * InputMethodManager.setInputMethodAndSubtype
 * InputMethodManager.switchToLastInputMethod
 * InputMethodManager.switchToNextInputMethod

In general, user mode apps should not have WRITE_SECURE_SETTINGS
permission.  Thus it might be not that difficult for us to simply
deprecate such a special rule.  Bug 114488811 is tracking that effort.

For now, this CL preserves the existing behavior when a null IME token
is specified to those 4 APIs.

Bug: 114418674
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: If762714b2003fa6477e1318110f63e13968c1d7e