[TODO] Android backport

[fython]

I use a lot of Java 8 API such as Stream, CompletableFuture, Optional which helps me build a beautiful code. But they're only supported since Android 7.0. If I want to adapt older Android, I may have to give up these API. That's too sad.

I found a compat library that takes lots of Java 8/9 API to old version: https://sourceforge.net/projects/streamsupport/

PS: it may induce package size a lot.

[fython]

Preview in floating window

We implemented it in a tradition way: WindowManager#addView with SYSTEM_OVERLAY / APPLICATION_OVERLAY flag.

• Support older version