Including OpenCV native library headers in Android Studio - opencv

Question

I can't get my C code using native OpenCV to work in Android Studio. OpenCV through the Java wrappers work, but I have been banging my head against this for the last couple of days trying to get it to work with the native libraries. (I have some C code that depends on OpenCV that I need to use in my project)
To pin down the problem I'm trying to get the tutorial-2-mixedprocessing OpenCV sample project to build in AS.
OpenCV is in a separate module in the project, with the shared libraries (.so, .a files) in jniLibs, and the headers are in jni. The application module has a dependency on it. See the image below for the sample code and result when building.
Apparently the headers are not found?
I have tried adding the -I flag to CFlags in android.ndk in the gradle file with no success:
CFlags.add("-I${file("path/to/opencv-android-sdk/native/jni/include/")}".toString())
I have also tried placing the headers in the app module jni folder, as well as using different versions of Android Studio and Gradle.
I keep finding outdated solutions and conflicting information online, it seems like the build system in Android Studio is still changing a lot between versions. I'm also new to both Android development and NDK, so apologies if this is a stupid question. All I really need is a working sample that I can adapt to my project.
What am I missing?
Project structure:
Project gradle:
// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
repositories {
jcenter()
}
dependencies {
classpath 'com.android.tools.build:gradle-experimental:0.7.2'
}
}
allprojects {
repositories {
jcenter()
}
}
OpenCV library module gradle:
apply plugin: 'com.android.model.library'
model {
android {
compileSdkVersion = 23
buildToolsVersion = "23.0.2"
defaultConfig {
minSdkVersion.apiLevel = 21
targetSdkVersion.apiLevel = 23
}
buildTypes {
release {
minifyEnabled = false
proguardFiles.add(file('proguard-rules.txt'))
}
}
}
}
Application module gradle:
apply plugin: 'com.android.model.application'
model {
android {
compileSdkVersion = 23
buildToolsVersion = "23.0.2"
defaultConfig {
applicationId = "org.opencv.samples.tutorial2"
minSdkVersion.apiLevel = 21
targetSdkVersion.apiLevel = 23
}
ndk {
moduleName = "mixed_sample"
//CFlags.add("-I${file("path/to/opencv-android-sdk/native/jni/include/")}".toString())
}
buildTypes {
release {
minifyEnabled = false
proguardFiles.add(file('proguard-rules.txt'))
}
}
}
}
dependencies {
compile project(':openCVLibrary2411')
}