fix(settings): paint the scan and keyboard notices before the work that blocks

on_tick runs before draw in the same pass, so a screen built for a blocking call was
never painted until the call returned -- on a panel ui.screen() had already cleared.
Scanning looked like a hang on a blank screen. The notice is now painted by the tap
itself and centered on both axes.

test/settings_busy.lua

@@ -0,0 +1,49 @@
+-- Run: lua test/settings_busy.lua
+-- The scan and keyboard screens announce themselves before work that blocks the loop for
+-- seconds. on_tick runs before draw in the same pass, so anything left for draw() to paint
+-- appears only after the blocking call returns -- on a panel ui.screen() has already
+-- cleared. These assert the announcement is painted by the tap itself.
+package.path = "sdcard/lib/?.lua;test/?.lua;" .. package.path
+
+local device = require("fake_device").install()
+
+dofile("sdcard/apps/settings/main.lua")
+
+local function tapRow(prefix)
+  device.painted = {}
+  draw()
+  for _, item in ipairs(device.painted) do
+    if item.label:sub(1, #prefix) == prefix then
+      device.painted = {}
+      on_touch_down(item.x + 2, item.y + 2)
+      on_touch_up(item.x + 2, item.y + 2)
+      return
+    end
+  end
+  error("no row labelled '" .. prefix .. "'")
+end
+
+local function paintedLabel(prefix)
+  for _, item in ipairs(device.painted) do
+    if item.label:sub(1, #prefix) == prefix then return item end
+  end
+  return nil
+end
+
+init()
+tapRow("wifi")
+tapRow("scan networks")
+
+-- Painted by the tap, with no draw() in between.
+local busy = paintedLabel("scanning")
+assert(busy, "the scan screen paints its notice before scanning")
+
+local width = #busy.label * device.charWidth
+local centerX = busy.x + width / 2
+local centerY = busy.y + device.fontHeight / 2
+local panelW, panelH = gui.getWidth(), gui.getHeight()
+assert(math.abs(centerX - panelW / 2) <= 1, "centered horizontally, got x " .. busy.x)
+-- The bar is above the app's frame, so the app's own height is what it centers in.
+assert(math.abs(centerY - panelH / 2) <= 1, "centered vertically, got y " .. busy.y)
+
+print("ok")