Unity: ignore mdb files and fix path to crashlytics file (it can be in other places)

Unity projects targeting the 2.0/3.5 runtime or built with mono < v5.0 generate `mdb` files, not `pdb` files.

Looks like the `crashlytics-build.properties` gets around in more than just the `StreamingAssets` folder, looking at [examples around the internets](https://github.com/auth0/sharelock-android/blob/master/app/src/main/assets/crashlytics-build.properties), so it should probably just be ignored as a filename.