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/core/res/AndroidManifest.xml b/core/res/AndroidManifest.xml
index 72863a2..9f2eef5 100644
--- a/core/res/AndroidManifest.xml
+++ b/core/res/AndroidManifest.xml
@@ -18,7 +18,7 @@
 */
 -->
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
-    package="android" android:sharedUserId="android.uid.system"
+    package="android" coreApp="true" android:sharedUserId="android.uid.system"
     android:sharedUserLabel="@string/android_system_label">
 
     <!-- ================================================ -->