SystemUI: more keyguard visualizer tweaks

- link and unlink in the background. this is an expensive operation and
  we should avoid running it on the main thread
- remove leftover getSystemService call
- keep track of whether QS is expanded to avoid starting visualizer when
  it is visible
- prefer light vibrant color from palette
- run palette generation async
- don't apply keyguard bitmap while visualizer is shown

Change-Id: I222a27df9438466423c63555041275e4bf153c64
Signed-off-by: Roman Birg <roman@cyngn.com>