Clean up ve.dm.SurfaceFragment#isolateAndUnwrap a bit

* Move tx variable into inner function
* Merge startOffsetChange and endOffsetChange into adjustment

Change-Id: I0f684ed11f89c067c9a20cd2c4eaf6b4aa50b8c7
Merge "Clean up ve.dm.SurfaceFragment#isolateAndUnwrap a bit"