Reuse death recipient of window session for its windows

The process of WindowState is impossible to be different from
the Session. Since Session has registered death recipient, it
is unnecessary to register for the windows belonging to the
same process.

This simplifies the code structure and reduces overhead to binder.

Additional note:
 WMS#addWindow has checked valid display at the beginning.

Bug: 163976519
Test: WindowManagerServiceTests

Change-Id: Id249c9b4d3f347383939ee70e0b0deb54be7172a