Release bitmap reference right after Per85ComputeTask done its job.

In some weird cases, AsyncTasks might not be released as soon as we
expect and the params might also be kept for a long time.

Therefore, we try to nullify the reference to the params right after
background job has been completed to avoid this kind of leakage.

Bug: 124127996
Test: run "atest android.app.cts.WallpaperManagerTest"
Test: Observe the memory usage during and after cts tests.
Change-Id: I1bb7fa827be84e1739ed47df521e687a3d7a1727