Camera2: fix long press shutter behavior

Long press shutter should perform auto focus and take the photo
when the shutter is released. This behavior is broken in
f5b1181157b6d7f65376c0068768f8f3f94b5d69 when longshot/burst mode
is added by CAF.

Fix by only setting the long click handler for longshot/burst when
it is enabled.

Change-Id: I5149134641ec790c03857b4ea23c43a9ff0adfda