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