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
Merge branch 'cyanogenmod' into cm-10.2