Camera2: Preview needs to be stopped when changing resolution

This was done in startPreview as a hack, but was removed in
f729b85e12cfa2674048d8cce2f724014b8fc910. Specifically add it
back when the resolution is changed.

Change-Id: I306abd788bafdff4e6fba7eec4e6ff05bb3cc47c
Merge "Camera2: Preview needs to be stopped when changing resolution" into alpha