Add swap operation to UniquePtr.

Enables swapping without mPtr ever becoming null. Required by
https://googleplex-android-review.googlesource.com/#/c/250540/

(cherry picked from commit be7922da0cf9b759f01f09b5f259e083f8d1f0b8)

Change-Id: I06ec54e174d251cd2938efba8e628d0eaf6ebcc5
1 file changed