Sanity-check backup agent name prior to instantiation

Fixes a crash that would occur if an app with a pending backup
pass in the pipeline was updated to remove its agent declaration
from the manifest (or other more esoteric ways that a backup
pass was expected to run for an app without their own agent).

Bug 5776591

Change-Id: I5a8bc8c12de6a2bfa82f5093fe3a15b754109ab1