Add backup-host module scaffolding & core JVM interfaces

Introduces the core contracts and annotations for the JVM host-side orchestrator of the Automated Backup and Restore testing framework:
- Configures the 'androidx.test.backup:backup-host' module and its build.gradle file.
- Declares the core 'BackupRestoreController' interface containing low-level primitive control methods (performBackup, performRestore, etc.).
- Introduces annotations and types for test suites configuration, transport emulation, and multi-device qualifier rules: 'BackupRestoreConfig', 'BackupTransportMode', 'BackupActionResult', 'Device', and 'Isolation'.

Relnote: N/A
Bug: 537118605, 519009084
Test: ./gradlew :test:backup:backup-host:assemble
Change-Id: I18c3c3a1a9bd26e9f9ae83ee58130b898cb52c22