android: a folder of audio files, and a paste of track links

The paste door now reads itself the way the desktop's does: one track link
in the paste and every line is looked up on its own public page
(playlist_import_track_links), which is the way past Spotify's 100-track
public ceiling — the wait screen counts them as they arrive. A fourth door
imports a folder of audio files off this phone, picked with the storage
plugin's own document-tree picker; playlist_import_folder grew a path
argument for it, because on Android it had no dialog to open and always
answered nothing. A YouTube playlist link needed no door at all — the link
one already dispatches it.
