Update javadoc for RUN_BACKUP_JOBS permission.

Instead of indicating the app must have a "primary" use case, instead,
mentioned that it must be a "major" use case instead to cover apps whose
primary function depends on backup.

Fixes: 323068649
Test: n/a (javadoc update only)
Change-Id: Iac87d8d27ee7b37deac3ac5c85cb37efe68d8fd0