Add an update.sh script

Add a script that downloads a new version of error_prone.  Also
switch the source URL to sonatype to match the URL that was used
in the internal version, and remove error_prone_annotations source
jar since it is unnecessary.

Bug: 64489631
Test: running update.sh with current version only updates last_upgrade_date
Test: running update.sh with new version updates all fields
Change-Id: If7ab1a88dc4f0e78c54c06e079bad9a0f3dd4581