Rework android support example

[lihaoyi]

Tries to address some of the issues in #3675

• Various tasks like sdkUrl or buildToolsVersion should be prefixed with android
• Hardcoded buildToolsVersion defaults to 35.0.0. I suggest to just remove the default.
• Hardcoded platformsVersion defaults to android-35. We could derive it from buildToolsVersion.
• In general lots of various paths which are just sub-paths of buildToolsPath have their own tasks. Instead we could have some "config" class, which returns common paths from a buildToolsPath, e.g. by applying a (version specific) layout/sub-paths.
• AndroidAppModule.androidSdkModule should have the type ModuleRef[AndroidSdkModule].
• androidResources hardcodes a path to src/main/AndroidManifest.xml. It should either lookup the file from sources task, or be it's own androidManifest: Source task.
• androidKeystore is probably meant to be a persistent task