Fix Android build warnings in VideoThumbnailPlugin.java ()

[seokangwoo9]

Previously Warning:

/.pub-cache/hosted/pub.dev/video_thumbnail-0.5.6/android/src/main/java/xyz/justsoft/video_thumbnail/VideoThumbnailPlugin.java uses or overrides a deprecated API.
/.pub-cache/hosted/pub.dev/video_thumbnail-0.5.6/android/src/main/java/xyz/justsoft/video_thumbnail/VideoThumbnailPlugin.java uses unchecked or unsafe operations.

Changes:

• Replace deprecated Bitmap.CompressFormat.WEBP with WEBP_LOSSY/WEBP_LOSSLESS on API 30+, safe fallback for older

-Remove unsafe raw/unchecked casts from MethodChannel arguments parsing

Verification:

• example/flutter build apk -debug succeeds without the previous warnings