libcamera: touch-to-focus fixes

- Do not send command to camera unless preview is started
  (fix 4.3 AOSP camera crash)
- Send touch AF command regardless of previous state
  (required to actually trigger touch AF at the new area)

Change-Id: I51a8f80ce1ec38f1052e4660f708be8649478a88
libcamera: properly advertise lack of scene mode

Otherwise Focal with errorneously show all available scene modes

Change-Id: Id2987c6f3a01a331485bb76d879a6178b1c22bf6