android: put the way out of a note back within reach

Moving the toolbar to the top took the back arrow with it, and left the only
exit from a full-screen editor in the top-left corner — the furthest point on
the display from a right-handed thumb, reached over the whole note to get to.
Reported on the first device pass, and correctly.

So the footer carries a Done as well as the timestamp. With the keyboard up it
sits directly above the thumb, which is where a hand already is for every other
part of writing a note.

The top-left arrow stays. Two affordances for one action is usually clutter,
but this is the case that earns it: the arrow is what habit, the system back
gesture and TalkBack all expect of a full-screen surface, and removing it would
strand the reflex to strike a duplicate costing one icon slot.

A word rather than a checkmark, on the same argument the overflow menu makes: a
tick in a notes app is a checklist item to anyone who has used one, and "Done"
cannot be misread, including aloud.

EditorSavedLine is now EditorFooter, since it is no longer only a line.

android/app/src/main/res/values/strings.xml

@@ -45,6 +45,7 @@
     <string name="editor_unsaved">Not saved yet</string>
     <string name="editor_edited">Edited %1$s</string>
     <string name="editor_just_now">just now</string>
+    <string name="editor_done">Done</string>
     <string name="editor_pin">Pin</string>
     <string name="editor_unpin">Unpin</string>
     <string name="editor_labels">Labels…</string>