Camera2: Fix ANR when switching camera during AF

- Check that 'mCameraDevice' is valid at beginning of
  setCameraParameters(). This will avoid null pointer
  exceptions at some cases, when setCameraParameters()
  is called by focus manager after closing camera.

CRs-Fixed: 668405
Change-Id: Iff4b81e725db6db11cdc205512b0fec1f71f333d