C2SoftFlacEnc: use calculatedAs to set read only fields

As per C2Config.h, C2ParamIndex kParamIndexMaxBufferSize is read-only.
Use calculatedAs instead of withSetter during its configuration

Bug: 411376155
Test: atest MctsMediaV2TestCases:AudioEncoderTest
Test: atest MctsMediaV2TestCases:CodecEncoderTest
Test: atest MctsMediaEncoderTestCases:EncoderTest
Test: atest MctsMediaV2TestCases:CodecEncoderBlockModelTest
Test: atest MctsMediaV2TestCases:CodecEncoderMultiAccessUnitTest
Test: atest MctsMediaV2TestCases:CodecEncoderBlockModelMultiAccessUnitTest
Test: atest VtsHalMediaC2V1_0TargetAudioEncTest
Flag: EXEMPT bugfix
Change-Id: Id90c2591e8deaaab240d095849fa6207da0dcb66