update.sh: Add option to update checkerframework jars.

Previously, update.sh updated javac/ and error_prone/, but
checkerframework needed to be updated manually.

This CL is a preparation step for updating error_prone to
2.1.3 in a future CL, once that version ibecomes available
on oss.sonatype.org.

Test: Checked that the following changed last_upgrade_date in
      {checkerframework,error_prone,javac}/METADATA but had
      no other effect:
      ./update.sh 2.0.19 9-dev-r3297-4 1.8.10
Test: Checked that the following results in what looks like the
      correct changes, including to soong/error_prone.go:
      ./update.sh 2.1.2 9-dev-r4023-3 2.2.2
Bug: 69485063

Change-Id: I8433be67c2edab7e69c3cce78e7833409e647109