[MM] Provides a non-null GestureContext during STL transitions

Utility that provides a GestureContext from the current
SceneTransitionLayout transition, using default values as a fallback if
the transition's own gesture context isn't available.

Test: Manually tested on Flexiglass and STL Demo app
Bug: 410524175
Flag: com.android.systemui.scene_container
Change-Id: I538f7da7b8dac671881a001c76b2288cd92058f4