Add jdk.internal.ref to package_allowed_list.txt (OpenJDK 11)

This is part of merging upstream changes from OpenJDK 11.28. New classes
have been added to core-oj, so there is a new path added to
package_allowed_list.txt:

 * jdk.internal.ref

Bug: 198792576
Test: atest CtsLibcoreOjTestCases
Change-Id: I19a40447bbb060612e0c2f3ef0770568345a7405