Support overriding APEX java contents

In addition to the current "apps", support overriding
bootclasspath_fragments, systemserverclasspath_fragments, and java_libs.

This is useful to build APEXes with slightly different contents, for
example where the base APEX could contain libraries that only use stable
APIs, but the overriding APEX could contain a variant that also uses
non-finalized APIs.

Bug: 222247869
Test: m (runs apex tests)
Change-Id: I14da6f835c58b44be31e517598695ec8ff830648