Fix IME Switcher menu starting position

This fixes the starting position when opening the IME Switcher menu.
The behavior is achieved through RecyclerView#scrollToPosition, however
when calling this before the first layout, it (incorrectly) scrolls
such that the selected position is at the top of the list.

This fixes it such that the selected position is at the bottom of the
visible list, by posting this call, such that it's run after the first
layout.

Flag: android.view.inputmethod.ime_switcher_revamp
Bug: 311791923
Test: open the menu with many subtypes enabled and observe
Change-Id: Ice2ee23280494c295e45519aec79eff6205d8a15