Add model to represent generated snapshot .bp file

Having each module type generate the contents of the snapshot's .bp
file results in lots of duplicated code. This adds an intermediate
model for use by the module types and then generates the .bp file
contents from that.

This not only removes the duplicated formatting code but it also
allows consistent handling of shared properties such as name further
reducing duplication. It also makes it possible to duplicate the
versioned and unversioned prebuilt modules from the same model.

Extracts generatedContents from generatedFile to allow the contents
to be populated without creating an output file, for testing.

Cleans up unused code.

Bug: 143678475
Test: m nothing
Change-Id: If21b84db0ef3fdfb5dc11ea0973ce6cb73603ea3