Fix VirtualDisplayTest by putting image checking task to an non-UI thread

The case failed on some devices w/ low end processor.
The root cause is that heavy loading image checking task running on UI thread make
the UI system cannot render desired color and pass check in 10s.

The fix suggestion is: put image checking task to an non-UI thread.

Change-Id: Iebc3ae594fefa1f8492800356f0a93f2c97fd5b7
Signed-off-by: Hurri Lu <jlu32@marvell.com>