Download and install desktop firefox on first-run of FxR Desktop

[patrickoshaughnessey]

We will only ship the Unity parts in the Viveport package, and on first-run, have it download and install Firefox (into C:\Program Files). Firefox will then update on it’s own on the normal release update cycle. If we update the Unity-only parts, users will get a notification via an in-app notification.

[patrickoshaughnessey]

First cut TODO

• [x] Move "StreamingAssets/firefox" contents (i.e. vrhost.*) into tools/bundle/firefox/overlay
• [x] Create new bitrise workflow that replaces "Inject Firefox Nightly" with "Copy stuff in "tools/bundle/firefox/overlay" to "StreamingAssets/overlay"
• [x] Add logic in Unity first run to detect installed Fx version, install/update if necessary, and copy StreamingAssets/overlay contents into Fx installation
• [x] Change hardcoded references to assumed StreamingAssets/firefox installation with detected installation location

[TomasEkeli]

Please don't hard-code in the location to "C:\Program Files", it's a sane default but users may want it somewhere else.