Fix flaky null pointer in LauncherWidgetHolder

- LauncherWidgetHolder.startListening is called before Launcher.setupView, so it's possible for getWorkspace() to be called as part of the appWidgetRemovedCallback. Reordered these calls to fix this null pointer

Flag: not needed
Fixes: 283850341
Test: Screenshot tests
Change-Id: Ied1f23647721fc4199eab2a5a79def1e306b00f3