Improve fallback logic in PinchInputReader.cs

[keveleigh]

• Changed hasPinchData to compare against the value instead, to better represent the Unity method IsInProgress() when going through the fallback path.
• Updated the polyfill logic to check if one of the "select" and "select value" is bound while the other is polyfilled. We can use the data from the bound action to synthesize the other better than the hand joint logic will. This also helps account for controller interaction profiles, like khr/simple_controller, that doesn't have both a bool and float action for select.

[keveleigh]

FYI @whebertML, wanted to make sure this doesn't affect the intent of #852