Camera: Modify previewEnabled() to check for overlay being valid.

previewEnabled should return TRUE if the preview is running. However
on 7630 when overlay is enalbed, this returns FALSE even though preview
is running because CAMERA_MSG_PREVIEW_FRAME is disabled after the
first callback. Hence add a check to see if overlay object is valid or
not.

Change-Id: I12284926e170bcb6865e27281fdc6d0820df16ec
Merge "Camera: Modify previewEnabled() to check for overlay being valid." into cm-10.1