Allow no item selected in IME Switcher Menu

This starts the checkedItem variable at -1, instead of 0, so when no
item is found to be checked we don't incorrectly show the first value as
selected.

This scenario can happen when we have a list of explicitly enabled
subtypes, which would no longer list the implicit subtype,
but the selected subtype is still the implicit one, which cannot be
found in the list.

Test: install a separate version of Gboard, disable and re-enable it
  from System Settings and switch to it, go to its app settings and
  add a new subtype, and show the IME switcher menu again. No items
  should be shown as selected.
Bug: 328579802
Change-Id: I431bdb0185b78be44b43a50910543f8b18a8c5ed