You should add following blocks into your Android.mk
LOCAL_STATIC_JAVA_AAR_LIBRARIES:= <aar alias>
.
.
.
include $(BUILD_PACKAGE)
include $(CLEAR_VARS)
LOCAL_PREBUILT_STATIC_JAVA_LIBRARIES := <aar alias>:libs/<lib file>.aar
include $(BUILD_MULTI_PREBUILT)
Please also be aware of satisfy minSdkVersion required by library into your manifest file.