systrace: avoid queuing signaled fence in FenceMonitor

If the HWC release fence dequeued has already signaled, the existing
mechanism will still queue it and show it later if there's release fence
waiting, which is not accurate. This change will avoid queuing signaled
fences into FenceMonitor to make the trace more reasonable.

Bug: 127781085
Test: build, flash, boot and take systrace with gfx category
Change-Id: Ie607f690bba8ccdfab4d02793abcf21e18c3eb63