Make MessageQueue inner classes package-private

Avoid the possibility of synthetic accessors being used to work around
access modifiers by relaxing access in source code.

When this code is compiled then synthetic accessors are stripped out.
However, recent experience tells us that this is not always dependable.

Flag: build.RELEASE_PACKAGE_MESSAGEQUEUE_IMPLEMENTATION
Bug: 415954362
Change-Id: I839ba829c4a02440786cfa3b506c2c20fa6441b8