as you're using Android Studio, your Makefiles are ignored by default and new ones are generated on-the-fly, without properly referencing OpenCV as it's not supported.

This is how NDK builds are currently working from Android Studio and it's deprecated while a better way to do it is in the work.

You can deactivate this built-in NDK support and get your Makefiles to be used instead, by doing this inside your build.gradle:

import org.apache.tools.ant.taskdefs.condition.Os

apply plugin: 'com.android.application'

android {
    ...

    sourceSets.main {
        jniLibs.srcDir 'src/main/libs' //set .so files directory to libs
        jni.srcDirs = [] //disable automatic ndk-build call
    }

    // call regular ndk-build(.cmd) script from app directory
    task ndkBuild(type: Exec) {
        if (Os.isFamily(Os.FAMILY_WINDOWS)) {
            commandLine 'ndk-build.cmd', '-C', file('src/main').absolutePath
        } else {
            commandLine 'ndk-build', '-C', file('src/main').absolutePath
        }
    }

    tasks.withType(JavaCompile) {
        compileTask -> compileTask.dependsOn ndkBuild
    }
}

btw, I see you set APP_ABI only to armeabi-v7a, but OpenCV also supports x86 and mips, so you can also easily extend your support to these platforms.