Create per-flavor findbugs tasks

[budnyjj]

It would be great to add per-flavor findbugs tasks to check only required classes
and don't depend on 'assemble' anymore, like it's done in:
https://github.com/ChaitanyaPramod/findbugs-android

Such measure should significantly decrease the build time.

[budnyjj]

See my patchset: #75

[vanniktech]

Closing this. Have a look at #75 for the reasoning behind this.