Codec2: fix API style for some helpers

- remove android::C2Component placeholders
- C2FSVQuery now has private instead of const members
- fix naming of helper classes and static methods
- document C2FieldSupportedValues better
- don't return rvalues to temporary objects

Bug: 64121714
Test: Build
Change-Id: I7a9c9f35fda294fd51578355050f043b92197720