Fix formatting of various backup source files

This is a pure refactor, the following changes were made:
- run `google-java-format --aosp`
- add curly-braces for if statements that don't fit on a single line
- fix function parameter comment style (e.g. /* field= */)

Bug: 403956528
Test: builds successfully
Test: atest FullBackupTest.java
Flag: EXEMPT refactor

Change-Id: I3661af90f42aaf4120ca94a83cf5bfc1322e9a90