PackageManager: Add configuration APIs for pre-launch checks

This change replaces overlays with an audited set() API, allowing
application with INTERCEPT_PACKAGE_LAUNCH permission to bootstrap
themselves.

Additionally, checks have been added to ensure that the custom prelaunch
check activity always resolves to a valid target.

Change-Id: I7f6be647fde3ce4ed699e6abfbe0392176ec96d3