Exclude jacocoagent from boot jars package check

This replicates the previous behavior when the boot jars package check
was performed by a singleton. When the singleton was removed and the
check was performed by the platform-bootclasspath the logic to exclude
jacocoagent from the list was lost. This change replaces it.

Bug: 191377804
Test: m EMMA_INSTRUMENT=true EMMA_INSTRUMENT_FRAMEWORK=true check-boot-jars
      - fails without this change passes with this change.
Change-Id: If9eae254045bef9a0c0963213721fc12a9f463cb