AR updates for Android

[nCastle1]

The changes in this PR should resolve the following issues:

• [ARToolkit] Black screen instead of camera on second and next usages of ARSceneView on select devices #382
• Replace SceneForm, which has been discontinued #379

Summary of changes:

• Update AR Core to 1.26.0
• Update SceneForm to 1.17.1 (last release before deprecation)
• Updates ARSceneView to destroy resources in OnDetachedFromWindow; testing shows that this seems to work well when navigating between activities within and app, and when switching between apps. This resolves an issue with the camera feed not rendering in some cases.
• Updates the hit test implementation, because hit testing stopped working with the new dependencies, possibly related to trackable is not a Plane xamarin/XamarinComponents#1305, but more testing is needed to confirm

Risks/things to look out for:

• hit test behavior has changed significantly; this more or less matches the native Android implementation, so it should be fine. It does look like results are less predictable in testing, but it isn't clear if that is driven by the hit test implementation changes, or changes to AR Core
• AR Core has updated plane detection and configuration options. In at least one test run, it was detecting vertical planes; I wasn't able to repro, but worth considering if vertical plane detection should be on by default.