Prevent duplicate IME callback registrations

This CL prevents the mImeCallbacks list in ImeOnBackInvokedDispatcher
from ever containing two callbacks with the same id.

Bug: 407224281
Test: Manual, i.e. verified IME back navigation works as usual
Flag: EXEMPT bugfix
Change-Id: I93e9ea44089710a7a9a3336dcd199b9fa0c166b0