Add a private API to set mocking hint manager for testing

Updated the doc of interval methods as they won't fail now

Bug: 346604998
Flag: EXEMPT ndk
Test: atest NativeSystemHealthUnitTest SystemHealthManagerUnitTest
            NativeSystemHealthTest HeadroomTest
Change-Id: Ia8a3d921f831ee0c07b16926500581fc2a8d5b00

Author: xwxw-g

include/android/system_health.h

@@ -417,7 +417,6 @@ __INTRODUCED_IN(36);
  * @param outMinIntervalMillis Non-null output pointer to a int64_t, which
  *                will be set to the minimum polling interval in milliseconds.
  * @return 0 on success.
- *         EPIPE if failed to get the minimum polling interval.
  *         ENOTSUP if API is unsupported.
  */
 int ASystemHealth_getCpuHeadroomMinIntervalMillis(int64_t* _Nonnull outMinIntervalMillis)
@@ -434,7 +433,6 @@ __INTRODUCED_IN(36);
  * @param outMinIntervalMillis Non-null output pointer to a int64_t, which
  *                will be set to the minimum polling interval in milliseconds.
  * @return 0 on success.
- *         EPIPE if failed to get the minimum polling interval.
  *         ENOTSUP if API is unsupported.
  */
 int ASystemHealth_getGpuHeadroomMinIntervalMillis(int64_t* _Nonnull outMinIntervalMillis)

include/private/system_health_private.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright (C) 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef ANDROID_PRIVATE_NATIVE_SYSTEM_HEALTH_H
+#define ANDROID_PRIVATE_NATIVE_SYSTEM_HEALTH_H
+
+#include <stdint.h>
+
+__BEGIN_DECLS
+
+/**
+ * For testing only.
+ */
+void ASystemHealth_setIHintManagerForTesting(void* iManager);
+
+__END_DECLS
+
+#endif // ANDROID_PRIVATE_NATIVE_SYSTEM_HEALTH_H
+