Extract constructLocaleFromString()

The main motivation of this refactoring is to move InputMethodUtils to
com.android.server.inputmethod as a package-private class.  With a lot
of cleanup CLs in Bug 77730201, the only remaining utility method that
is used by multiple Java package is this constructLocaleFromString()
method.

This is purely a mechanical refactoring.  There should be no
observable behavior difference.

Bug: 114660660
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Test: atest FrameworksCoreTests:com.android.internal.inputmethod
Change-Id: Ic47b680ad9efde104015d34311f49e224d3fb56f