Fix IME callback not unregistered when IME is up during an IMM client

switch.

This can happen when one navigates from an activity showing IME
to another activity requesting to show IME. The following sequence would
happen:
- IMM of the current activity receives an UNBIND message with reason CLIENT_SWITCH
- A new IMM is created for the new activity requesting input and
  requests to bind.
- The new IMM requests to start input, triggering
  IMS#dispatchStartInputWithToken with a *new* ImeOnBackInvokedDispatcher instance than the one previously kept by IMS.

Previously, the ime dispatcher instance was updated before the existing
back callback was unregistered and caused the unregister call to fail.
The fix is to update the dispatcher after the other logic in
IMS#dispatcherStartInputWithToken (specifically, to after startInput(),
which calls doFinishInput() if input is already started).

Bug: 234416098
Test: atest CtsInputMethodTestCases

Change-Id: Ic7a5daab8709b808343388b095b5b6ad40adc777