android samples: update way to attach native part of OpenCV

OPENCV_ANDROID_SDK - OpenCV Android SDK location (or OpenCV build dir)
Can be passed via ndk-build parameter or environment variable.

samples/android/face-detection/jni/Android.mk

@@ -5,7 +5,15 @@ include $(CLEAR_VARS)
 #OPENCV_CAMERA_MODULES:=off
 #OPENCV_INSTALL_MODULES:=off
 #OPENCV_LIB_TYPE:=SHARED
-include ../../sdk/native/jni/OpenCV.mk
+ifdef OPENCV_ANDROID_SDK
+  ifneq ("","$(wildcard $(OPENCV_ANDROID_SDK)/OpenCV.mk)")
+    include ${OPENCV_ANDROID_SDK}/OpenCV.mk
+  else
+    include ${OPENCV_ANDROID_SDK}/sdk/native/jni/OpenCV.mk
+  endif
+else
+  include ../../sdk/native/jni/OpenCV.mk
+endif
 
 LOCAL_SRC_FILES  := DetectionBasedTracker_jni.cpp
 LOCAL_C_INCLUDES += $(LOCAL_PATH)