Add support for Android <= 7.1.1

[bm2ilabs]

This pull request for the issue related to #187
Failed to load dynamic library (dlopen failed: library "libobjectbox-jni.so" not found)

[greenrobot-team]

Thanks. Could you provide more details on why you think the added code is necessary to support Android 7.0 and older?

    private static void loadLibrary() {
        if (isLibraryLoaded) {
            return;
        }

        // Loads `libobjectbox-jni.so`.
        System.loadLibrary("objectbox-jni");
        isLibraryLoaded = true;
    }

Update: tested our example on an Android 5.0 emulator and it runs fine without this change.

[greenrobot-team]

We shipped an implementation of this, we welcome feedback if it works: #369 (comment)