[TEST] Mostly standalone TFLM

[veblush]

This PR transitions TFLM toward a standalone repository structure by migrating TFLM-owned components under tensorflow/lite/micro/ and maintaining only canonical upstream LiteRT C API files in tensorflow/lite/.

• Summary of Changes
  • Kernels & Reference Math: Moved tensorflow/lite/kernels/internal/ implementations to tensorflow/lite/micro/kernels/internal/ and scoped internal reference functions under namespace tflite::micro.
  • Tools & Types: Moved flatbuffer utilities, Python schema generation, and half.h under tensorflow/lite/micro/.
  • Removed Shims: Deleted legacy forwarding shims from tensorflow/lite/kernels/ and tensorflow/lite/experimental/.
  • LiteRT Parity: Verified all tracked LiteRT C API and schema files in ci/tflite_files.txt match upstream byte-for-byte (0 diff).
  • Includes & BUILD Rules: Updated internal kernel includes and BUILD targets across micro/, signal/, and third_party/hexagon/.

bug=n/a