backup: no-op code cleanup

1. remove unused methods,imports,members
2. make members private/final as much as possible

Bug: 122371936

Test: 1) atest RunBackupFrameworksServicesRoboTests
2) atest $(find \
frameworks/base/services/tests/servicestests/src/com/android/server/backup \
-name '*Test.java')
3) atest CtsBackupTestCases
4) atest CtsBackupHostTestCases

Change-Id: Ibd37ae124be87487b0b30bcb92b012c1713f6555