Removes strong ref to Activity from RequestFinishCallback

This change eliminates the strong reference from
RequestFinishCallback to Activity.

Bug: 139782368
Test: Open then finish an activity within an ongoing app process
      Observe the Activity instance is no longer strongly reachable
      in the app process after onDestroy() completes.

Change-Id: I13e9707e96721a2d491629147337aaedc3473651