Remove pid record mapping after killing process

Some methods may enforce the caller belongs to a valid process record.
If the record is removed before killing the process, there could be
a rare timing that the access of the process is considered as invalid.

Fix: 386548874
Flag: EXEMPT bugfix
Test: Hardcode delay:
       SystemClock.sleep(SystemProperties.getInt("test-kill-delay", 0));
       at the beginning of WindowManagerService#openSession
       and before app.killLocked in removeProcessLocked.
       adb shell setprop test-kill-delay 2000
      Cold launch the target activity.
       adb shell am stop-app $pkg
      There is no exception when creating window Session from the app.

Change-Id: I4f440de6579e32dc5c6a885a0cc7243ba54695f4