Define separate DeliveryGroupMatchingKeys for Dropbox Broadcasts

Dropbox sends two DROPBOX_ENTY_ADDED broadcasts at the same time to
handle a CompatChange. The two broadcasts need to have different
DeliveryGroupMatchingKeys or one of them will get replaced by the other.

Fixes: 323450448
Test: atest CtsDropBoxManagerTestCases
Change-Id: Ibdf7439cf5eb86b0c267ebe7ddc6962912dcdfd6