Refactor emulated_camera to use shared generic implementation with CaptureDeviceFormat trait

Extract `EmulatedCamera` and `EmulatedCameraSession` structs and their
`VirtioMediaDevice` and `VirtioMediaIoctlHandler` implementations into the shared
`vhu_media` library to eliminate duplication.

Introduce the `CaptureDeviceFormat` trait in the shared library to abstract
format-specific differences between `emulated_camera_mplane` and
`emulated_camera_splane`.

Key changes:
- Created `CaptureDeviceFormat` trait in `vhu_media`.
- Moved `Buffer`, `BufferState`, `EmulatedCameraSession`, and `EmulatedCamera`
  to `vhu_media`, making the camera and session generic over
  `F: CaptureDeviceFormat`.
- Moved and unified `VirtioMediaDevice` and `VirtioMediaIoctlHandler`
  implementations to `vhu_media`.
- Refactored `emulated_camera_mplane` and `emulated_camera_splane` to:
  - Implement `CaptureDeviceFormat` for `MplaneFormat` and `SplaneFormat` respectively.
  - Implement `default_fmt` and `write_pattern` directly within the trait
    impl blocks.
  - Instantiate the generic `EmulatedCamera` in `main.rs`.

TAG=agy
CONV=b4f9ffcf-c393-4dbc-b8fb-5c38416291c2

Bug: 519646531
Test: cd base/cvd && bazel build cuttlefish/package:cvd
      # Repeat with v4l2_emulated_camera_mplane
      cvd create -media=type=v4l2_emulated_camera_splane
      # Ensure pixel format changes with each type
      v4l2-ctl -d1 --all
      # Ensure it passes with no failures
      v4l2-compliance -d1 -s

Author: Brian Daniels