Steps to Reproduce:
- Set up an Oculus Rift
- Use Editor 2019.2.0a2
- Open the attached reproduction project "TPD Test.zip" with
- Open SampleScene.unity
- Press Play
- Hide a controller from the tracking cameras.

Expected Behavior:
When camera tracking is lost for a controller the position should remain at its last known good value.

Observed Behavior:
When camera tracking is lost for a controller the position returns to the scene origin. This is most easily observed through the scene view.

Tracked Pose Driver should be using TryGets for position and rotation. If position is not updated by the sdk, the last known good values should stick.

*Note* 2018.3 still uses TrackedPoseDriver in managed distributed C#, rather than TPD from the XR Legacy Helpers package.