Remove deprecated Metadata interface from camera-camera2-pipe

Remove the legacy androidx.camera.camera2.pipe.Metadata interface and
its Key class from camera-camera2-pipe now that all usages across AndroidX
and Google3 have migrated to the unified androidx.camera.common.Metadata.

- Delete camera-camera2-pipe/src/main/java/androidx/camera/camera2/pipe/Metadata.kt
- Update FrameMetadata and RequestMetadata to extend androidx.camera.common.Metadata directly
- Update CameraMetadata and CameraPipeKeys to reference androidx.camera.common.Metadata.Key
- Update FakeMetadata and tests in camera-camera2-pipe and camera-camera2-pipe-testing

Test: ./gradlew :camera:camera-camera2-pipe:test :camera:camera-camera2-pipe-testing:test :camera:camera-camera2:test
Change-Id: I58506a0d593a9256008e0c627fcacb47878f135b