Hardware decode: MediaCodec backend (Android, HEVC + AV1)

[justin13888]

• HwStillDecoder impl over NDK AMediaCodec, cfg-gated target_os = "android".
• HEVC: build csd-0 (Annex-B parameter sets) from HevcConfig; convert payload to Annex-B.
• AV1: csd-0 from av1C; device-codec capability probe.
• Map YUV output buffers (Flexible/P010) to DecodedFrame.
• Fix and document the minimum API level (HEVC vs AV1 availability differ).
• Emulator CI smoke test if feasible; otherwise fixture-driven plumbing tests.
• Depends on: Create rawshift-hwdec (HEVC+AV1); HEIC on gamut-heic; drop libheif-rs #27.