Add API for virtual preloads

Virtual preloads are applications that aren't actually on the
/system partition, but, act as if they were. One such distinction
is that these apps receive Intent.ACTION_BOOT_COMPLETED and start
out of the stopped state.

Change-Id: I812d3e7008b9d87e84aa33dbc4b3d8e8b334533c
Fix: 34855677
Test: Manual
Test: Install an app with "--preload"
Test: See that it receives Intent.ACTION_BOOT_COMPLETED