update_engine: Remove release/aquire semantic from scoped_ptr/unique_ptr

Now that scoped_ptr is just a type alias to unique_ptr, there is no
longer a need to convert between the two by using .release() and
constructing the other with a raw pointer.

BUG: None
TEST: Built for dragonboard, all unit tests pass (except for update_engine,
      but those tests failed before these changes too).

Change-Id: I0afcbc4d85b7ca78149fb29c188d7bcc99d4c80b