Android support revised (SCL-6273) + tests fixed

Based on request #5 support of android-sdk-plugin. Related changes in intellij-scala will be pull requested a bit later.

Differences from #5:
- no android-sdk-plugin dependency for sbt-structure (anyway we can't use it while calling sbt-structure from IDEA)
- removed hardcoded properties and changed xml syntax
- possible to extend for other android plugins as well