The entry page of Android's DRM documentation [1] says that Android provides a DrmManagerClient-native implementation as the interface to native modules. From what I can tell, the interface is DrmManagerClient.h [2].
When it comes to the Java API, it's clearly stated that only API level 18 and higher supports the modular version of Widevine. API 18 introduced the MediaDrm class, while the NDK contains a NativeMediaDrm.h from API level 21 on. Newer Androids comes with "libwvdrmengine.so" and "libwvm.so", which could be one library for WV modular and one for WV classic. An Internet search shows libwvm.so being used on Android 4.0.4 and later.
Which Android-versions support Widevine-modular in DrmManagerClient-native?
[1] https://source.android.com/devices/drm.html
[2] android/platform/frameworks/av/include/drm/DrmManagerClient.h