fix(heif): detect HEIC and AVIF motion photos without libheif

[DichenZhang1]

Summary

Mirrors Google3 cl/990031203 (fixes b/567336720): enables the Java API (MetadataEngine.parseMetadata, parseMetadataFd, isMotionPhoto), Android builds, and C++ ParseMotionPhotoFrom*(..., nullptr) to detect HEIC and AVIF motion photos without requiring libheif.

Root Causes Addressed

1. JNI parse entry points previously hard-coded FileType::kJpeg; ParseMetadataFromFd now sniffs the ftyp box header via SniffFileType().
2. MetadataEngine::Parse previously had no container splitter for HEIF, producing an empty MetadataCollection.
3. C++ ParseMotionPhotoFrom* previously detected HEIF only via MotionPhotoHeifInfoBuilder when out_metadata != nullptr (and only when compiled with libheif, which is disabled on Android).

Changes

• container_splitter.h / jpeg_container_splitter.cc / heif_container_splitter.cc / metadata_engine.cc:
  • Renames CreateContainerSplitter() in jpeg_container_splitter.cc to CreateJpegContainerSplitter().
  • Adds CreateHeifContainerSplitter() in heif_container_splitter.cc, which reads the ISO BMFF item boxes of the meta box directly (pitm, iinf, iref, iloc, idat) and emits the primary image's XMP item as a standard.xmp block when a valid top-level mpvd box is present.
  • Dispatches CreateJpegContainerSplitter() vs CreateHeifContainerSplitter() in MetadataEngine::Parse().
• motion_photo_jni.cc: Adds SniffFileType() and uses it in ParseMetadataFromFd and AGTM entry points.
• heif_container_splitter_test.cc / libmotionphoto_api_test.cc / MetadataEngineTest.java / lib/core/CMakeLists.txt: Adds unit tests for HeifContainerSplitter, ParseMotionPhotoFrom*(..., nullptr), and Java MetadataEngine HEIC detection.