In PHP 8.1 the default $flags argument to htmlspecialchars() has changed
from ENT_COMPAT to ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML401. This
breaks some tests.
I changed all the calls that break unit tests, and some others
based on a quick code review. A lot of callers just use the default for
convenience, and were already over-quoting, so the default should still
be good enough for them.
Change-Id: Ie9fbeae6f0417c6cf29dceaf429243a135f9fecb
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
