overlay: Advertise camera button presence for qwerty devices

- xt897 is the only moto_msm8960 family member with physical
  camera button while it's also the only qwerty device, so let's
  indicate the camera button presence based on the qwerty
  availability
- while this is not always correct in the case of an external
  keyboard connected to a non-qwerty moto-msm8960 device,
  it covers the majority of use cases quite well anyway

Change-Id: I7e1a8ac6a7bc00736ff838e39f08d30a40d6392e