Add "Open AVD Manager" to device selector

[kenzieschmoll]

From discussion on #3146

@stevemessick:

we might want to see about adding "Open AVD Manager" to the device selector, for compatibility with Android Studio. AS also has "Troubleshoot device connections" under it, which might be worth investigating for inclusion in Flutter projects.

[stevemessick]

#3146 was kind of a red herring, it is really old. The AVD manager works fine in recent builds, and already has a button on the toolbar. We could do as this issue says to make it possibly easier to discover, but it isn't that important.

[pq]

Closing as stale as per Steve's assessment. We can re-open as needed.