Delegate retrieval of dex boot jar for apex to the bootclasspath_fragment

The dex boot jar for the apex must have had hidden API flags encoded
into it. Currently, the hidden API processing is done within the java
modules themselves so the apex gets the dex boot jar from them.

However, as part of the hidden API modularization work the hidden API
encoding will be performed by the bootclasspath_fragment so this change
prepares for that by delegating the retrieval of the dex boot jars to
the bootclasspath_fragment, via BootclasspathFragmentApexContentInfo.

For the moment that simply delegates straight back to the java module
so this change does not change the build. It will however make it
easier to switch hidden API encoding to the bootclasspath_fragment in
future.

Bug: 179354495
Test: m com.android.art
      - verify that this change does not change its contents
Change-Id: I12eba333749be976bcc72661bb9d6be6cc3c56e3