Load back-gesture model on background thread.

Loading the back-gesture model and its assets on the UI thread can
block the UI thread for ~100 ms. This change frees up the UI thread
by loading the model on a background thread. Back-gesture detection
can still occur while the model is loading.

Fix: 218329819
Test: atest OnBackInvokedDispatcherTest BackAnimationControllerTest BackNavigationControllerTests
Change-Id: If222311318c96a8b1d27916ccebf83697e3d9ad1