Do not bind IME until an IME client is recognized

This is an attempt to simplify InputMethodManagerService (IMMS) for
better maintainability.

Currently there are three cases where IMMS binds to the current
InputMethodService.

 A. When the system becomes ready to run 3rd party apps.
 B. When the user is switched to another user.
 C. When an IME client is recognized while no IME is bound to IMMS

The idea of this CL is to remove A and B.  At a first glance, the
conditions A and B seem to make sense, because it sounds like
pre-warming InputMethodService to reduce the cold startup latency.

However, the problem of A and B is that IMMS#mCurClient can be null.
This means for instance we need to have a special fallback logic such
as using default display to start InputMethodService.  Also, most
likely we would have the same pre-warming effect even without A and B,
because once an IME client is recognized by IMMS, IMMS just starts
binding to the current InputMethodService anyway even if the active
IME client does not have text edit field.

Therefore this CL removes A and B in favor of simplicity.

Fix: 117730713
Test: manually tested on both initial boot and user switch scenarios.
Change-Id: I52f6c4cd1e02be3a59e9a87e33b0a44f4ba8d80b