Codec2: Combine C2ParamField and C2FieldSupportedValues

In many constructs these go together. Create a new combined
structure. Also create a specific structure for querying supported
values as we can avoid the unique_ptr and unnecessary allocations
in that case.

Also fix some minor bugs in C2AvcSoftDec.

Bug: 64121714
Test: unittest
Change-Id: I59da175329c11269dc0c28dc03475701cda2118d