CtsVerifier: Camera FOV test: Fix crash from double-tapping preview

The onClick callback for the preview view has no guard on checking
whether the camera is still active and preview is running, it just
issues a takePicture unilaterally.

This doesn't work if multiple onClick() callbacks are queued up and in
between processing them, a picture arrives and the activity is paused.

Guard the takePicture call with a check on whether preview is still
active.

Test: Not able to crash CtsVerifier by tapping quickly on FOV test
      preview
Bug: 37911172

Change-Id: I6902d383bcb970b27e48a2b4854cb459e7a33658
1 file changed