Mark com.android.server.textservices.LocaleUtilsTest as @SmallTest

This is a follow up CL to my previous CL [1] that extracted a utility
method getSuitableLocalesForSpellChecker() from InputMethodUtils to a
newly created LocaleUtils class.

One thing I forgot was copying @SmallTest from the previous test case
to the new test case.  Other than restoring @SmallTest, this CL
changes nothing.

 [1]: If6cd051aa0ae7780631d5bec5b7d01faac4a83a0
      3781ca8c

Bug: 115516399
Test: atest FrameworksServicesTests:com.android.server.textservices
Change-Id: Iad12aa2a022e8c41150c6b2d52bea670981c4446