Move logic for backup journal into its own class

Refactor to further simplify BackupManagerService, which currently has too
many responsibilities. Also adds unit tests.

Bug: 36850431
Test: adb shell am instrument -w -e package com.android.server.backup com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: Id433d7604c22c8b6f0d524a9bf9e83053facc0ca