Fix TTS for GMT offset

Add TtsSpans to the GMT offset string so that TalkBack knows to
read it out in a more natural way.

Retain the old keys with string types for compatibility with TV and
wear.

Test: m SettingsLibTests && \
      adb install -r $OUT/data/app/SettingsLibTests/SettingsLibTests.apk && \
      adb shell am instrument -w com.android.settingslib/android.support.test.runner.AndroidJUnitRunner
Bug: 30042703
Change-Id: I5629a0113404b32b606c1f03060f32868b59c6d2