Make the caching logic use getWithSetCallback() and simplify
the code given that there is only one Preprocessor subclass.
Also, keep the cached values JSON serializable but rely on
the serialization in BagOStuff instead for simplicity.
Add related class constants for injecting preprocessor flags.
Bug: T254608
Change-Id: I72f9f0c0bc352ed5120469090c71294ff0c24999