Debugger: `j` opens debugger on most recently connected target (#45060)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/45060

Currently, `j`, (i.e., `/open-debugger` with no parameters), connects the "first available" target, which in practice is the first page of the first connected device still connected.

In the absence of a target selection UI, a better guess at user intent is to use the *latest* target (most recently added page of most recently connected device).

Also slightly reduces CLI noise by not claiming that we're launching a debugger when there's no target, and not qualifying which target when there's only one.

Changelog:
[General][Changed] Debugger: `j` opens most recent (not first) target.

Reviewed By: huntie

Differential Revision: D58736151

fbshipit-source-id: 3d106a1fa958f9e5c91b16e04075609e1abf6e97