Codec 2.0: fix multi-fd sync fence support

Fix multiple issues with multi-fd sync fences:
- add expanded documentation for public methods
- restore single-fd native handles to have a single fd
- use proper magic number in the native handles, and
  distinguish ordered and unordered multi-fd fence native
  handles to ensure APIs behave as described.
- fix handling all previous layouts of native handles

Add unit tests to ensure compatibility.

Bug: 326528469
Test: /data/nativetest64/codec2_vndk_test/codec2_vndk_test and high-level tests
Change-Id: I5123dbd51dfd94c5f821e9663a0674a0a6e73b22