Add missing test runner annotations

These tests are supposed to run even withotu @RunWith annocation, but
for better consistency we should have it there.

Bug: 110805255
Test: verified as follows. No new test failures.
    tapas adb LatinIME LatinIMETests arm64 userdebug && \
    DISABLE_PROGUARD=true make -j LatinIME && \
    adb install -r $OUT/system/app/LatinIME/LatinIME.apk && \
    atest LatinIMETests:com.android.inputmethod.latin.ContactsDictionaryUtilsTest && \
    atest LatinIMETests:com.android.inputmethod.latin.ContactsContentObserverTest
Change-Id: I5c115d57b2ad41642f5f9b09b0c0d8f8c12480b0