Export implementation class jars for java_boot_libs

Hiddenapi processing currently requires access to the class
implementation jars for libraries on the bootclasspath which means that
they need to be provided as part of the prebuilts. This change modifies
the java_boot_libs property on the sdk to make those files available.

Modularization of the hiddenapi processing will hopefully remove the
need for these to be exported so this should be temporary.

Bug: 178361284
Test: m art-module-sdk
      check generated snapshot zip contains implementation jars
Change-Id: I9e94662dddb0ddb85a477ae6d27e533085147e88