Codec2: clean up base vs. baseIndex confusion

- rename baseIndex to coreIndex
- define ParamIndex
- change names of macros defining base structs from '_NO_BASE' to
  'BASE'
- fix creating flexible params from base structs and add unit tests
  to verify

Bug: 64121714
Test: unittest
Change-Id: I95c237a4328e5313604588b695e2fd117001dda3