Fix formatting and style of PackageManagerBackupAgent

This is a pure refactor. The following changes were made:
- run `google-java-format --aosp`
- move some non-javadoc method comments inside that method
- add curly braces where required
- add @Override annotations for `onBackup` and `onRestore`.

Bug: 277594991
Test: m services.backup
Change-Id: I091586d1d1b8f008e6d5a6262b5916eb8998b7dc