Make sure JNI_OnLoad is not called more than once

Since Android 5.0 Google introduce a nasty bug[1] which calls
JNI_OnLoad more than once.
Basically every time when a library is loaded JNI_OnLoad is
called if found, but it calls *again* JNI_OnLoad of its .so
dependencies!

[1] Workaround https://code.google.com/p/android/issues/detail?id=215069



Change-Id: I34728572bc279581b8c95c42c206ed104bfc09a5
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/plugins/sensors/android/src/androidjnisensors.cpp

@@ -241,6 +241,11 @@ static bool registerNatives(JNIEnv *env)
 
 Q_DECL_EXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void * /*reserved*/)
 {
+    static bool initialized = false;
+    if (initialized)
+        return JNI_VERSION_1_6;
+    initialized = true;
+
     typedef union {
         JNIEnv *nativeEnvironment;
         void *venv;