Make getLanguageFromLocaleString private

As part of effort to lock down the use of InputMethodUtils only within
the core system components, this CL removes the dependency on
InputMethodUtils#getLanguageFromLocaleString() from the Settings UI
code.

At high level, there should be no user visible behavior change.

Bug: 77730201
Test: atest SettingsLibTests:com.android.settingslib.inputmethod.InputMethodSubtypePreferenceTest
Change-Id: I92cc0b0808f3e40ab92b60ec783ad218141f2bbd