Enable omitting IME Switcher Menu header

This updates the IME Switcher Menu header logic to omit the title for
groups of items with only a single item without a subtype name.
Additionally, if all the items belong to the same input method, no
header will be shown.

Flag: android.view.inputmethod.ime_switcher_revamp
Bug: 369120217
Test: atest InputMethodMenuControllerTest#testGetMenuItemsNoHeaderForSingleInputMethod
  InputMethodMenuControllerTest#testGetMenuItemsHeaderTitles
Change-Id: I977831b2020fb9c92b4a20a8e8fb86cb512a8a43