Camera2: fix start preview when hdr is on issue

when we have already set hdr on and do not set off, we don't need
to set it to scene mode to auto every time. This will make the camerea
to start preview again, which will make the app freeze for a while.

Change-Id: Ica77106b88056e289cd298eda4ccc5c3f4f6ab59
CRs-Fixed: 679200