Fix keyguard wallpaper issues

- separate file observers for proper bookkeeping
- cache keyguard wallpaper in PhoneStatusBar so we don't query it and
  generate unnecessary binder calls every time updateMetaData() gets
  called (which happens a lot)
- more consistency with regular wallpaper logic

REF: NIGHTLIES-1429, CYNGNOS-386
Change-Id: I0bf7d5ffdf8972cf10ba1a90ae9c5d3f9a6a9aab
Signed-off-by: Roman Birg <roman@cyngn.com>