Check window alive by death recipient of window session

The isBinderAlive only takes effect if a death recipient is
registered and sent or it has performed a transaction which
gets DEAD_OBJECT status (BpBinder.cpp).

Because no one register death recipient to mClient, its
isBinderAlive usually returns true even if the process is died.

Bug: 403551058
Flag: EXEMPT bufgix
Test: Hard code to not clear DisplayPolicy#mNotificationShade
      in removeWindowLw and kill SystemUI. The restarted
      SystemUI won't crash by validateAddingWindowLw.
Change-Id: I9b77601a01825130ed1ac594fc34005b7e13e58d