Fix startup jank in the Wifi Picker.

Synchronously fetch the latest APs during onResume and show these before
displaying the other wifi preferences. This fixes a visibile shuffling
issue. Additional Setting preferences are now programatically added
during onStart and hidden during onStop in order to avoid the animation
delay which causes a shuffling effect.

Bug: b/36406983
Test: Visual inspection. Animation jank is difficult to programatically
test.

Change-Id: I34b2d737a8e72e1dc296c170f753975fafce7a3e