Adapt to latest ddmlib version

This change is a quick adaptation to changes in DDM lib used by Android Gradle plugin 0.14.x.
Closes #208.

That said, I do not have a strong leg to stand on what assumption failure really means. However, it seems to correspond to `error` in previous versions.
I didn't add ignored test handling since it requires additional changes in reports code.

I tested this with combination of spoon gradle plugin and Android plugin 0.14.1.
There's been a JUnit 4 runner in AOSP for about 6 months now. https://android.googlesource.com/platform/frameworks/testing/+/master/androidtestlib/