Add more @NonNull/@Nullable to TextServicesSettings.

We need to fix TextServicesSettings as well as InputMethodSettings,
because regarding Bug 27687531 the same issue was copied from
InputMethodSettings to TextServicesSettings by my CL [1].

This is basically a mirror of the fix for InputMethodSettings [2].

  [1]: Ie3d61458648df469abe149b7aaad8087c531a675
       49ed1401
  [2]: I169ad4957e68b65c64251b0849056195b8ca4911

Bug: 27687531
Change-Id: I068f8821a32d2e199fbf6239bbe833da8c5a4e5e