Stop assuming language part in locale string is up to 2-letter

With this CL, InputMethodSubtypeSwitchingController stop
assuming that language part in locale string is up to 2-letter.

this CL is not so risky because
InputMethodSubtypeSwitchingController have used language part for
nothing but sorting subtypes to determine the initial rotation
order of IME switching.

BUG: 16502987
Change-Id: Ib159ece0b7aa04f8fd7abc96c9a6e0832a4b7cd2