Remove an unnecessary int to String conversion.

This is a safe refactoring to remove an unnecessary int to String
conversion in TextServicesSettings.

Settings.Secure.SELECTED_SPELL_CHECKER_SUBTYPE is a integer value that
indicates subtype ID (or SpellCheckerSubtype#hashCode() if the subtype
ID is not specified), and we can just rely on
Settings.Secure#putIntForUser() rather than converting int to String
by ourselves then pass it to Settings.Secure#putStringForUser().

Note that this change is still OK for existing users because
Settings.Secure#putIntForUser() has been internally doing exactly the
same thing.

Bug: 27687531
Change-Id: Ibcf12746f1295c12bec095300ea7f6ced0a51d09