Use classpath elements in platform_bootclasspath

Use classpath elements in newMonolithicHiddenAPIInfo. That means the
method can collate information from both fragments and libraries rather
than just fragments. So, this change moves the collation of the
classesJars into the method.

Bug: 177892522
Test: m out/soong/hiddenapi/hiddenapi-flags.csv out/soong/hiddenapi/hiddenapi-index.csv
      - make sure that this change does not affect the contents.
Change-Id: I7c2a229fab60d02bd211438735a8d7303ed83386