Camera2: add option to prevend autofocus cancel if preview stopped

There are camera HALs which cannot handle the autofocus cancel call if the preview
stopped in startPreview(). Add an option for this HAL's to do not cancel autofocus in this cases.

Change-Id: I0767cc7279405d17afc0b00120cb43e5bb309220

Conflicts:
	res/values/config.xml
	src/com/android/camera/util/CameraUtil.java