Convert Tensor from Java to Kotlin (#19823)

Summary:
Pull Request resolved: #19823

Convert `Tensor.java` to idiomatic Kotlin while preserving full JNI and Java interop compatibility:

- `abstract class Tensor` with `protected constructor` for subclass access
- `JvmField DoNotStrip val shape` for direct JNI field access
- `JvmStatic` on all companion factory methods (`fromBlob`, `ones`, `zeros`, `numel`, `allocate*Buffer`)
- `DoNotStrip` on JNI-called methods (`nativeNewTensor`, `dtypeJniCode`, `getRawDataBuffer`)
- Nested tensor type classes (`Tensor_uint8`, `Tensor_int8`, etc.) with `internal constructor`
- `halfBitsToFloat` moved to `Tensor_float16.Companion`, using `Float.fromBits()`
- `when` expressions replace `switch` statements and `if-else` chains
- Kotlin array initializers for `ones()`/`zeros()`
- Updated BUCK to reference `.kt` source

Differential Revision: D106557156

Author: kirklandsign

extension/android/BUCK

@@ -14,7 +14,7 @@ non_fbcode_target(_kind = fb_android_library,
         "executorch_android/src/main/java/org/pytorch/executorch/ExecutorchRuntimeException.kt",
         "executorch_android/src/main/java/org/pytorch/executorch/MethodMetadata.kt",
         "executorch_android/src/main/java/org/pytorch/executorch/Module.java",
-        "executorch_android/src/main/java/org/pytorch/executorch/Tensor.java",
+        "executorch_android/src/main/java/org/pytorch/executorch/Tensor.kt",
         "executorch_android/src/main/java/org/pytorch/executorch/annotations/Experimental.kt",
     ],
     autoglob = False,