Pose flicker fix on some devices

The issue was that when the sensord starts on some devices, the initial
IMU sensor fusion pose timestamps are 0 (the first 20-30). It then
starts sending the timestamps. But the latency model looks into the
latency in the initial samples and fixes the latency estimate.

Therefore the estimated latency was way off.

The solution is checking the pose timestamps before using it.

Bug: 37278649
Test: Restart device and run a VR app.
Change-Id: Ic472cb43b32de5d4af3b19547bdc32e906fbfa6a