libcameraservice: Workaround for nvidia TTF bug

When leaving from a TTF scenario to a non-TTF (such as complete lack
of focus on camcorder or FFC), the focus areas are reset to 0
Setting a TTF region of size zero will cause the whole HAL to lock up,
so prevent it from happening...

Change-Id: I21886a2c8665a50aa6f9ff92a6aaf1f6c36ad61c