Move boot jars package check into platform_bootclasspath

Bug: 177892522
Bug: 189298093
Test: m check-boot-jars
      m SOONG_CONFIG_art_module_source_build=false check-boot-jars
      - Ran both commands with and without java.lang in the
        package_allowed_list.txt
Change-Id: Iba1a881c8f6b6919d5c0c0520eb3073658f3b8d2