SurfaceFlinger: Fix sourcing sync from external display

- Vsync was not switching to external displays when the system property
  "vendor.display.pluggable_vsync_prioritized" was set.
- Trigger switch of vsync when external display is powered on.
- Update active vsync source when the external display is un-plugged.
- Modify getVsyncperiod() to return vsync period of current or
  next active display which will source vsync.

Change-Id: Idc1cd4101913c8d97385c82a164bf022bd79e14e
Crs-fixed: 2579886