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
Camera2: tweak volume key zoom and cleanup

- cleanup unused code
- cleanup codestyle
- add more zoom steps for smother zooming
- show zoom circle indicator during zoom

Change-Id: I5214fc2620a0c5167bfdc8e85f7115966c3b8bcf