Optimize @Stable field for Method.isCallerSensitive

JDK-8271820 introduced the `@Stable private Boolean callerSensitive;` cache in `Method`. That field is essentially a tri-state `Boolean` that relies on its `null` value to serve as "not initialized" value for `@Stable`.

This works well when the holder `Method` instance is constant: JIT compilers fold `@Stable` well. But if such folding fails, we always dereference the full-blown reference to "boxed" `Boolean` from the field on every access. We can instead do a more lean tri-state primitive field to improve that non-optimized case without sacrificing optimized case.