Hide IInputMethodManager.{add,remove}Client() from apps

Since IInputMethodManager.{add,remove}Client() are just callbacks from
WindowManagerService to InputMethodManagerService, we can simply move
those two methods to InputMethodManagerInternal instead of having them
in IInputMethodManager.aidl, which ends up exposing those methods to
any random user process.

This enables us to clean up WindowManagerService because it is no
longer responsible for obtaining IInputMethodManager and passing it to
each Session instance.

This also allows us to get rid of several RemoteException handlers
from Session class, because now the complier knows that those
callbacks will never throw RemoteException.

Fix: 112670859
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Test: prebuilts/checkstyle/checkstyle.py -f frameworks/base/services/core/java/com/android/server/inputmethod/InputMethodManagerInternal.java
Change-Id: I453200fd5847e9a78876affb6a1caec221525e1d