Fix some keyguard wallpaper issues

Partial cherry pick of Change-Id:
I0bf7d5ffdf8972cf10ba1a90ae9c5d3f9a6a9aab

- Cache wallpaper in PhoneStatusBar so we don't query it every time
  media meta data tries to update

- Don't crash if service isn't connected yet, return null

Change-Id: I8f0c8a53b634db5ffd27be3975b7b2db1b35bbfb
Signed-off-by: Roman Birg <roman@cyngn.com>