Root cause: SpannableStringBuilder cannot handle the preference title is
empty caes.
Solution: Assign default value in test case setup.
Bug: 188448957
Test: make RunSettingsRoboTests -j52 ROBOTEST_FILTER=AccessibilityButtonFooterPreferenceControllerTest
Test: make RunSettingsRoboTests -j52 ROBOTEST_FILTER=AccessibilityControlTimeoutFooterPreferenceControllerTest
Test: make RunSettingsRoboTests -j52 ROBOTEST_FILTER=CaptionFooterPreferenceControllerTest
Test: make RunSettingsRoboTests -j52 ROBOTEST_FILTER=ToggleAutoclickFooterPreferenceControllerTest
Change-Id: Ia95a6ecba824d81e0ccf649975517231ecf475b3