This results in a slightly larger apk (e.g. 500KiB), but it reduces the
scope for bugs greatly. We still get all the benefits of only having to
maintain a single density-independent vector (rather than several
density dependent PNGs and all the work that involves).
The class of bugs that it solves is that there are several places where
vectors cannot be used, and you wont notice when developing on a device
newer than 5.0. For example, notification icons, `TextView`
and its `android:drawableStart` attribute.
Fixes#913.
