Fix a pseudo race condition in TtsEngineSettingsFragment.

The issue is that Fragment.setVoiceDataDetails can
be called before Fragment.getView. We guard against this
issue. No thread visibility issues here because both functions
are called on the UI thread.

bug:5884355
Change-Id: Iad91b91c58b04dcb9f34f6b5ff8752f2e8295423