Fix memory leak caused by mismatched linkToDeath() in WindowManagerService

This fixes a bug where an allocated DeathRecipient in WindowManagerService
was holding a reference to keyguard binder interface after a call to
linkToDeath() without a matchin unlinkToDeath().

It was causing the keyguard side of the binder interface to stick around,
which in tern prevented the keyguard side from releasing its references.

The solution is to ensure matching linkToDeath()/unlinkToDeath() calls.

Fixes bug 11982048

Change-Id: I6959816b819ba953512c53675162195cbf1e0653