android: [render-validation] support multiple backends

[poweifeng]

This commit introduces several features:

• Backend Testing: Support testing across both OpenGL and Vulkan. Test configurations expand per backend, and the Filament Engine alongside ModelViewer is recreated dynamically when the backend switches to isolate resources.
• UI Controls: Added a backend filter toggle and a test progress bar to the main activity, themed appropriately using Material design.
• Memory Optimizations: Drastically reduced memory usage by displaying scaled-down thumbnails (128x128) in the main list. Full-resolution images are lazy-loaded from disk on demand when clicking a thumbnail to view, and large bitmaps are aggressively recycled.
• Device Info Collection: ValidationResultManager now collects and reports detailed GPU driver info for each instantiated backend in the output results.json.
• Engine Teardown: Introduced an explicit destroy() method in ModelViewer to ensure full resource cleanup during engine recreation.