By allowing plugins to reuse DelegateRepository functionality
multi-site and high-availability plugins can avoid code
duplication and significantly reduce the code complexity.
Also, make the annotation @UsedAt repeatable for associating
the DelegateRepository functionality with the usage from multiple
plugins.
Bug: Issue 13429
Change-Id: I2b5f5b215395fc1ef2a8008a71f5c09278d1278b