Fix vararg build warning

Fix the following build warning:
[...]/InputMethodAndSubtypeUtil.java:399: warning: non-varargs call
of varargs method with inexact argument type for last parameter;
    ListFormatter.getInstance(locale).format(subtypeNames), locale);
                                                   ^
cast to Object for a varargs call
cast to Object[] for a non-varargs call and to suppress this warning

Test: Existing tests pass (cd tests/robotests && mma)
Change-Id: Ie13ff713d3dbbe1f308fcd105656a484b89c8f1d