Docs: Updating Doze/App Standby docs from Preview to 6.0 release

This change updates this page from M Preview 3 to official 6.0 release.
As part of this update, it also relocates relevant material from the
previously existing "Testing" page that was part of Preview.

Bug: 23725346

Change-Id: I3a875dcca6620956c5fbbff5610faa47ca7747e6