Initialize CloseGuard for legacy synthetic DexFiles

[kasnder]

Problem

The pre-Android O generated-dex fallback allocates dalvik.system.DexFile with AllocObject so
it can attach an in-memory ART dex. On API 21-25 this bypasses the constructor's initialization of
the private CloseGuard guard field. Finalization later calls close() and throws a
NullPointerException from CloseGuard.close().

Fix

Resolve CloseGuard.get() and the legacy DexFile.guard field during DexFile::Init, then seed a
fresh guard on every synthetic object returned by ToJavaDexFile. If any framework contract is
missing, initialization fails closed and LSPlant remains unavailable on that runtime.

Evidence

• HookBridge ARM64 release native build passes.
• A source contract regression test covers class/method/field lookup and object seeding.
• With the fix enabled on an API 21 ARM64 emulator, LSPlant initialized and installed its hooks;
  the six fixture logs contained no DexFile.finalize/CloseGuard.close NPEs. The broader fixture
  harness still reported no-bound rows, so this is not presented as a complete compatibility pass.