probeWallpaperHash
Hashes a sample of the current system wallpaper (Android 7+).
Reads up to the first 4 KB of the wallpaper file and SHA-256 hashes it, giving a stable fingerprint of the user's wallpaper without exposing the image.
Privacy: the wallpaper is personal content; only a hash of a byte sample is retained. On newer Android versions reading the wallpaper file may require READ_EXTERNAL_STORAGE, hence this is an optional probe that fails soft.
Return
SHA-256 hex of the wallpaper sample, or null if unavailable.
Parameters
context
Context used to obtain WallpaperManager.