merge master into fad/next (#4102)

* Deflake firebase_common HeartBeat tests. (#4083)

The tests relied on `TestOnCompleteListener` that was not safe to call
more than once since it was based on a count down latch. So reusing it
multiple times would cause await() to return immediately.

This change makes it so that a new latch is created for every await()
call, making all await() calls work.

Fixes: http://b/245956774

* Add Javadoc support to the DackkaPlugin (#4082)

* Add util method for copying directories

* Add javadoc support to our dackka plugin

* Remove the extension check on fromDirectory

* Add a note about cache compliance and the javadoc task

* Add reference to kotlin stdlib package list (#4093)

Co-authored-by: Vladimir Kryachko <vkryachko@google.com>
Co-authored-by: Daymon <17409137+daymxn@users.noreply.github.com>