Codec2: move global variable declarations behind a macro flag

Use #define __C2_GENERATE_GLOBAL_VARS__ prior to including headers
to generate globals. This is currently used for C2Param fieldList
member initialization.

TODO: investigate if initializer lists can be returned by
inline static class methods instead that don't require this
separation, and if they produce no memory overhead.

Bug: 64662308
Change-Id: I7050182bf756aae66d7cfc2785472046e93eb7c8