Implement the import and rendering of glb files

Add an import FloatingActionButton which allows the import of binary glTF files (*.glb)
- The import mechanism makes use of content providers on android
Imported *.glb files are then rendered onTap of the AR plane
- However, only one model at a time can be 'active'
- Additionally, since importing uses content providers, each import switches out of the Palantir app, so the scene is lost each import
- The chance of recovering the scene when control is returned to Palantir is very low
- So, effectively, each import clears the scene
Add a TextView to indicate the current selected *.glb file
- TextView shows 'null' when no *.glb file is 'active'
- Palantir will also show a Toast prompting the user to import a file, if an AR plane registers a tap, but there is no imported *.glb file

ning-y / Palantir