ApexCodecs: ApexCodec_Configurable_config semantics change

|written| is renamed to |writtenOrRequested| and will contain the
requested size of the buffer in case the given |config| had
insufficient size. |config->size| does not change as a result.

In addition, add _Nullable/_Nonnull annotations.

Bug: 297922713 
Bug: 380300185
Test: presubmit
Flag: EXEMPT NDK (APEX native)
Change-Id: I08144239bebe63467148c0e53130cf9dc044b2cc