Refactorings around usage of jfieldID

- Make encoding and decoding require the mutator lock, in preparation
  for next CL.
- Store jfieldID instead of ArtField* in jvmti data structures, to avoid
  needing the mutator lock for certain operations.

Test: test.py
Bug: 481456995
Flag: EXEMPT PURE_REFACTOR
Change-Id: I88950b8d732db805ef23011e3f9557575cca686a
8 files changed