Summary
-------

Clarify the behavior of `java.lang.reflect.AccessibleObject::setAccessible` and  `trySetAccessible` APIs when invoked by JNI code with no Java frames on the stack that it only allows access to public member of a public type that is unconditionally exported per the access check as described in the class specification.

Problem
-------

The current implementation of `AccessibleObject::setAccessible` and  `trySetAccessible` throws NPE when invoked by  JNI code with no Java class on the stack.  The behavior of `AccessibleObject::setAccessible` and  `trySetAccessible` should be consistent with the access check.

The class spec of `AccessibleObject` has been updated in Java 13 (JDK-8221618) that JNI code with no Java class on the stack can only access public members of a public type that is in a package that is exported unconditionally.   

Solution
--------

Fix the implementation of `setAccessible` and `trySetAccessible` to allow setting the accessible flag on public members of a public type that is exported conditionally when invoked by a native thread attaches to the VM with no caller frame.    Also clarify the specification of `canAccess`,  `setAccessible` and `trySetAccessible` w.r.t. when invoked by JNI code with no Java frame on the stack.

Specification
-------------

`AccessibleObject::setAccessible(boolean)`

```
      *     open module. </li>
      * </ul>
      *
+     * <p> This method may be used by <a href="{@docRoot}/../specs/jni/index.html">JNI code</a>
+     * with no caller class on the stack to enable access to a {@link Member member}
+     * of {@link Member#getDeclaringClass() declaring class} {@code D} if and only if:
+     * <ul>
+     *     <li> The member is {@code public} and {@code D} is {@code public} in
+     *     a package that the module containing {@code D} {@link
+     *     Module#isExported(String,Module) exports} unconditionally. </li>
+     * </ul>
+     *
      * <p> This method cannot be used to enable access to private members,
      * members with default (package) access, protected instance members, or
      * protected constructors when the declaring class is in a different module
```

`AccessibleObject::trySetAccessible`
```
+     * <p> If this method is invoked by <a href="{@docRoot}/../specs/jni/index.html">JNI code</a>
+     * with no caller class on the stack, the {@code accessible} flag can
+     * only be set if the member and the declaring class are public, and
+     * the class is in a package that is exported unconditionally. </p>
+     *
```   

`AccessibleObject::canAccess`

```
-     * with the variation noted in the class description. </p>
+     * with the variation noted in the class description.
+     * If this method is invoked by <a href="{@docRoot}/../specs/jni/index.html">JNI code</a>
+     * with no caller class on the stack, this method returns {@code true}
+     * if the member and the declaring class are public, and the class is in
+     * a package that is exported unconditionally. </p>

```