Kotlin in SystemUI

Queue "it's happening" gif.

Kotlin is probably going to be a bit of a wild west for a while, but please
try to follow these guidelines as much as possible.

• No semi-colons: they are optional, we probably don't want them in the
  future, so let's just not add them.
• No DSLs: sysui is complicated enough as is, let's not add more layers at
  the moment.
• Only use extension functions for keeping complex code locality: Don't use
  extension functions to add methods to android classes that you always wished
  were there, instead add them directly to the class and save us the extension.
• inline, reified, and de-compisition can all be great things: just make sure
  you know what they do and why you are using them.

Recommended reading

• Kotlin
• AndroidX-KTX
• Performance and Kotlin tricks