Fix TalkBack shortcut keeps showing enabled when entering into that page.

Root Cause: TextUtils.isEmpty() will return true both null and empty case, and write config key into it.

Solution: Write config key only if settings key is null. That means no one (developers or users) touch the key ever, so we should respect config key here.

Bug: 255912415
Test: make RunSettingsRoboTests ROBOTEST_FILTER=ToggleFeaturePreferenceFragmentTest
Change-Id: I092975ea3de8fce78b7ae2a6241a30f6af8125a6