Split IME Switcher menu header and item

This extracts the header (and divider) part of the IME Switcher menu
items from the actual item layout, enabling more granular control on
when and where headers should be added.

Flag: android.view.inputmethod.ime_switcher_revamp
Bug: 369120217
Test: atest InputMethodMenuControllerTest
Change-Id: Ia6b0f35be96f252ab97c380c5d8984210a15a936
7 files changed