Force the build tools to generate .pngs for each vector.

Merged username-removed-25042 requested to merge pserwylo/fdroidclient:fix-913--vector-issues into master

This results in a slightly larger apk (e.g. 500KiB), but it reduces the scope for bugs greatly. We still get all the maintenance 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 (closed), which is coming up a few times in ACRA crash reports.