[FBR] Extract app metadata backup to helper

In full backup, we backup additional metadata about the app
(manifest, widget, apk, obb) not specified by the app's backup agent.
This CL extracts these methods out to their own helper (AppMetadataBackupWriter)
and adds unit tests for these methods.

** Note: The backup behavior is the same, only the structure has changed.
Behavioral changes will be done in future CLs. **

What this CL covers:
- Move the backup of this extra app data out of the FullBackupEngine to
separate agent data backup and non-agent data backup.
- Move logic of deciding what data to backup from FullBackupEngine to
FullBackupRunner (where the writer is used).
- Add unit tests for metadata backup.
- Some style fixes/clean up.

Not covered (future CLs):
- Refactoring FullBackupEngine/FullBackupRunner mechanism.
- Streaming backup data directly instead of writing to temporary files.
- Separating out and fixing apk and obb backup.

Bug: 110081582
Test: 1) atest AppDataBackupWriterTest
2) atest RunFrameworksServicesRoboTests
3) atest GtsBackupHostTestCases
4) Verify success for:
 - adb shell bmgr backupnow <full backup package>; adb restore 1 <full
backup package>
 - adb backup <full backup package>; adb restore
 - cloud backup and restore
5) Use local transport and adb backup to inspect manifest and widget data
written and file metadata consistent between runs.
6) Verify compatibility with adb backup -keyvalue manifest

Change-Id: Icb43fd2e0505c2416738ee3ef370b206363fac68