Allow shared libraries on bootclasspath

A previous change treated this as an error in order to try and detect
issues with misconfigured java_sdk_library modules and/or
bootclasspath_fragment modules but unfortunately this is not always an
error, e.g. when migrating a library that was a shared library to the
bootclasspath.

This change stops treating that as an error.

Bug: 179354495
Test: m nothing
Change-Id: I4a833ab5f4caf86c6cd340090fc65d2c2f141512