Camera2: add exposure time support

Use existing slow shutter infra from n1 but support
using exposure-time parameter

Enabled if max-exposure-time param is available

TODO:
maybe oppo special - traced from color os
a value of 1 is auto
a value of 0 is disabled

-stop countdown timer on second press on shutter button
-use count down timer also for slow shutter
-fix update of burst mode image

PS3
-added 32s

Change-Id: I1ece9ae0e2b38547cbba4b9d8ea2830463c05ff2