Extract createEntriesForApexFile

While this seems like a simple refactoring it does actually fix an
issue. Previously, when the ExtraEntries func was being defined inline
within the for ... loop it used the loop variable "fi". Unfortunately,
that meant that the func created on each iteration ended up using the
value for "fi" that was set on the last iteration.

Extracting the creation of the entry into a separate method means that
each func created gets its own "fi" variable with the correct values.

Bug: 177892522
Test: m SOONG_CONFIG_art_module_source_build=false nothing
      - without this change it reported duplicate rules for apache-xml.jar
        which is the last entry in the art-bootclasspath-fragment. With
        this change it works fine.
Change-Id: Ia063b513f758e1bbe73407c0441b72453f412acf