Fix formatting in backup and restore sources and tests

This is a pure refactor, just running `google-java-format --aosp` on
these files and some minor changes like adding annotations and updating
javadoc to make checkstyle happy.

Bug: 432673356
Test: builds successfully
Test: atest ActiveRestoreSessionTest.java (both of them)
Test: atest FullRestoreEngineTest.java
Flag: EXEMPT refactor

Change-Id: Icd43f5f06917815ede970ccac1fc63b5465adc36