Fix NPE when popping ANR dialog for system

The process record of system server is created before
initializing window manager. Then its wm internal is
always null.

Bug: 117877476
Bug: 147912509
Test: Simulate ANR in system server:
      Install a test app that contains a no response receiver
      declared with running in system server.
      Use command "shell am broadcast --receiver-foreground ..."
      to start the receiver and check ANR dialog is popped.

Change-Id: If41370e7e0ef799bbd4055d73e020bd3871df092