Fix formatting of various backup source files

This is a pure refactor, the following changes were made:
- run `google-java-format --aosp`
- fix function parameter comment style (e.g. /* field= */)
- add curly-braces where required
- add `m` prefix to class variables (required by pre-upload hooks)

Bug: 403956528
Test: builds successfully
Test: atest BackupAgentTest.java
Flag: EXEMPT refactor
Change-Id: I0ca8f32ed8c7eb2111c6d84f92029805af7044e0