Add WTF when a system server wrapper can't be found

2nd try -- enable it only within the system server.

This is to detect someone trying to access system services too
early during a boot.

Bug: 149406139
Test: Boot with "adb logcat | grep -w SystemServiceRegistry" and make sure no wtf is logged
Change-Id: I64c6776cedaeb534d98621db2955d585e6846b8d