Some clean ups for the 3P C++ API's Gradle / CMake integration.

In particular:

- Use `list(PREPEND ...)` rather than `set(...)` to prepend to `CMAKE_MODULE_PATH`.

- Clarify the distinction between `tflite_cc_DIR` and `tflite_cc_INCLUDE_DIR`,
  and use the former rather than the latter for the call to `add_subdirectory`.

- Define `tflite_cc_api_DIR` in the same place where `tflite_cc_INCLUDE_DIR`
  is defined, i.e. in `Findtflite_cc_api.cmake`, rather than in a different file.

- In `Findtflite_cc_api.cmake`, verify that the appropriate files exist in
  the specified directory, and report error(s) if they don't.
  (If CMake is invoked via Gradle, then the Gradle file
  should already ensure that those files exist there, by unpacking the
  contents of the AAR file and copying the `cc_sdk` directory there;
  but having explicit checks here means we'll fail
  earlier, and hopefully with a better error message, if CMake is invoked in some
  other way or if future changes break things.)

PiperOrigin-RevId: 706640601

Author: fergushenderson

lite/examples/native_api/android_play_services/cc_api/app/src/main/cpp/CMakeLists.txt

@@ -47,12 +47,11 @@ find_package(tensorflowlite_jni_gms_client REQUIRED CONFIG)
 #-----------------------------------------------------------------------------
 # Set up TFLite in Play services C++ API (tflite_cc_api) dependency.
 
-set(CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/Modules" ${CMAKE_MODULE_PATH})
-set(tflite_cc_api_DIR "${CMAKE_SOURCE_DIR}/tflite_cc_sdk")
+list(PREPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/Modules")
 
 find_package(tflite_cc_api REQUIRED MODULE)
 include_directories(${tflite_cc_api_INCLUDE_DIR})
-add_subdirectory(${tflite_cc_api_INCLUDE_DIR} tflite_cc_api_build)
+add_subdirectory(${tflite_cc_api_DIR} tflite_cc_api_build)
 
 #-----------------------------------------------------------------------------
 # Set up compile definitions to enable use of TFLite in Play services

lite/examples/native_api/android_play_services/cc_api/app/src/main/cpp/Modules/Findtflite_cc_api.cmake

@@ -1,5 +1,25 @@
 # This CMake code fragment determines the location of the tflite_cc_api package.
-set(tflite_cc_api_INCLUDE_DIR "${CMAKE_SOURCE_DIR}/tflite_cc_sdk")
+
+# tflite_cc_api_DIR is the directory containing the CMakeLists.txt file
+# for building the TFLite-in-Play-services C++ API client SDK source files.
+if (NOT EXISTS "${CMAKE_SOURCE_DIR}/tflite_cc_sdk/CMakeLists.txt")
+        message(SEND_ERROR "CMakeLists.txt not found in ${CMAKE_SOURCE_DIR}/tflite_cc_sdk")
+elseif (NOT EXISTS "${CMAKE_SOURCE_DIR}/tflite_cc_sdk/tensorflow/lite/abi/cc/interpreter.cc")
+        message(SEND_ERROR "tensorflow/lite/abi/cc/interpreter.cc not found in ${CMAKE_SOURCE_DIR}/tflite_cc_sdk")
+else()
+        set(tflite_cc_api_DIR "${CMAKE_SOURCE_DIR}/tflite_cc_sdk")
+endif()
+
+# tflite_cc_api_INCLUDE_DIR is the include directory containing the header
+# files for the TFLite-in-Play-services C++ API.
+if (NOT EXISTS "${CMAKE_SOURCE_DIR}/tflite_cc_sdk/tensorflow/lite/interpreter.h")
+        message(SEND_ERROR "tensorflow/lite/interpreter.h not found in ${CMAKE_SOURCE_DIR}/tflite_cc_sdk")
+elseif (NOT EXISTS "${CMAKE_SOURCE_DIR}/tflite_cc_sdk/tensorflow/lite/abi/cc/interpreter.h")
+        message(SEND_ERROR "tensorflow/lite/abi/cc/interpreter.h not found in ${CMAKE_SOURCE_DIR}/tflite_cc_sdk")
+else()
+        set(tflite_cc_api_INCLUDE_DIR "${CMAKE_SOURCE_DIR}/tflite_cc_sdk")
+endif()
+
 include(FindPackageHandleStandardArgs)
 find_package_handle_standard_args(tflite_cc_api DEFAULT_MSG
-                                  tflite_cc_api_INCLUDE_DIR)
+                                  tflite_cc_api_DIR tflite_cc_api_INCLUDE_DIR)