[Bugfix] Overlapping key gesture handlers: check existing pid alive

Binder linktoDeath() callback might occur after the sysui process is
restarted in case of an overloaded system. Check if the binder for
the existing handler is still alive before throwing exception.

Bug: 414477543
Flag: com.android.window.flags.delegate_back_gesture_to_shell
Test: manual by not listening to binderDied callback and killing sysui
Change-Id: Ib9885c249968f4fb8dc32e4744618179e981f1fa