Refine backup eligibility for non-main HSUM users

This change restricts TelephonyProvider backup eligibility to only
the main user in an HSUM environment.
BackupEligibilityRules now uses a new helper method to dynamically
select the correct system package allowlist. This differentiates
between the HSUM main user, profile users, and other non-system users,
ensuring TelephonyProvider is excluded for non-main HSUM users.

Added unit tests to confirm behavior for HSUM main/non-main users.

Flag: android.multiuser.backup_activated_for_all_users
Bug: 417649298
Bug: 406114361
Test: atest BackupEligibilityRulesTest
Change-Id: Idb1d83ef677b8422a02b38d291fb7fa7e56ffe0c