Add boot mode where only "core apps" are started.

A core app is one that has coreApp="true" in its manifest tag.

The system can successfully boot (though a little painfully) with
only framework-res.apk, SettingsProvider.apk, SystemUI.apk,
DefaultContainerService.apk, and Launcher2.apk set as core apps.

Currently this boot mode is always turned off.

Change-Id: Ieaa4a8031c2c391a12996aa8d8b1d65fc2b09d6b
diff --git a/services/java/com/android/server/SystemServer.java b/services/java/com/android/server/SystemServer.java
index 2714fc5..7e28c4f 100644
--- a/services/java/com/android/server/SystemServer.java
+++ b/services/java/com/android/server/SystemServer.java
@@ -148,7 +148,8 @@
 
             Slog.i(TAG, "Package Manager");
             pm = PackageManagerService.main(context,
-                    factoryTest != SystemServer.FACTORY_TEST_OFF);
+                    factoryTest != SystemServer.FACTORY_TEST_OFF,
+                    false);
 
             ActivityManagerService.setSystemProcess();