Hold onto linked binder.

unlinkToDeath is no longer required, (this change in behavior is to
avoid leaks) so holding onto window manager here (still calling
unlinkToDeath to avoid a log, but may for instance remove all
unlinkToDeath calls in the future).

Bug: 134576445
Test: boot
Change-Id: I78259964b564d87c4bb9be254ee46d3ce04db5ad