Bug 890427: use camera API's onPreviewStateChange callback to avoid deadlock. r=johu

do the same thing in loadCameraPreview
(cherry picked from commit 376d45c7afdbf97f75d55803e15fbb87931f3a50)

Conflicts:

	apps/camera/js/camera.js
Merge pull request #11821 from davidflanagan/bug890427-v1-train

Bug890427 v1 train