Codec2Client: misc bug fixes for ApexCodec

- Fix flush handling
- Fix buffer-too-small error handling
- Fix param reflector
- Remove `thread_local`s
- Refactor param blob parsing

Bug: 435023366
Flag: EXEMPT bugfix
Test: MctsMediaV2TestCases MctsMediaDecoderTestCases
Change-Id: Icfcb06ee4f58c1b37d76c871e2a95b82623a2eda