Camera3: Only add faceDetect metadata when necessary
Camera3: Only add faceDetect metadata when necessary

Only if number of faces detected are greater than 0, add
faceDetect metadata.

Change-Id: I99715a64c7014d9d50aeededf8ccf597a84910ad

maruos/android_device_lge_hammerhead,maruos/android_device_lge_hammerhead,maruos/android_device_lge_hammerhead,maruos/android_device_lge_hammerhead