Refactor AppBackupUtils into BackupEligibilityRules

AppBackupUtils contains static utility methods for checking backup
eligibility of packages. Refactor it into an instance class called
BackupEligibilityRules to better reflect the responsibilities of the
class, encapsulate eligibility-related state as well as logic and
simplify testing. See the bug description for full rationale.

Bug: 161241479
Test: atest AppBackupUtilsTest
Change-Id: I8ae47ac2a15e3c70da8feab752dd8354951224ea