Cache Java codegen'd flags in static member variables.

By caching flag values directly in member variables instead of caching
a HashMap and accessing that, flag reads avoid `hashCode()`, map
lookup, and Boolean.parse runtime costs. Flag reads are turning out
to have performance problems in hot paths, so this should help to
alleviate that.

Bug: 309625014
Test: m
Change-Id: I923bf6af2ae3fcbbf2fee7126b492a47cd6049ad