Call into application to do AppZygote preloading.

For now, assume a class named <package_name>.ZygotePreload
exists, with a method named doPreload() to do the preload.

This should probably be replaced by having an entry in
the manifest.

Bug: 111434506
Test: preload called
Change-Id: Id632e46087eb5c719e6b310ffe007f76d3cbaf7d