Fix ar-hit-test component not working since A-Frame 1.6.0

[vincentfretin]

Description:

Testing on https://aframe-xr-starterkit.glitch.me/ AR mode where we had the shadow of the piano in A-Frame 1.5.0
with a message at the bottom "Scanning environment, finding surface."
It doesn't work since A-Frame 1.6.0 because of commit 77af389 that changed the order of setting the ar-mode state so that particular check in the ar-hit-test component

aframe/src/components/scene/ar-hit-test.js

Lines 273 to 275 in 388eb8c

	this.el.sceneEl.renderer.xr.addEventListener('sessionstart', function () {
	// Don't request Hit Test unless AR (breaks WebXR Emulator)
	if (!this.el.is('ar-mode')) { return; }

broke because ar-mode state is not set at the time this sessionstart listener is called.

Changes proposed:

The fix is here to change
this.el.sceneEl.renderer.xr.addEventListener('sessionstart'
to
this.el.addEventListener('enter-vr'
similar to what we do in the reflection or hide-on-enter-ar components.

[dmarcos]

Thanks!