Allow app to specify class name for app zygote preloading.

Instead of using a fixed class name, define an interface for
preloading application code that must be implemented by applications
that use the application zygote.

Also, add an ApplicationInfo parameter to said function, so the app
knows where to look for its data and code.

Bug: 111434506
Test: atest CtsApptestCases:ServiceTest
      atest CtsSeccompHostTestCases

Change-Id: I1f8472da89dc90562dcb4e479e3d87ebf49b926c