This adds the default shortcuts and actions for keyboard-based focus traversal of apps.
This list of shortcuts includes shortcuts for TAB, SHIFT TAB, RIGHT_ARROW, LEFT_ARROW, UP_ARROW, DOWN_ARROW, and the four DPAD keys for game controllers (because the DPAD produces arrow key events).
It doesn't yet include functionality for triggering a control (e.g. SPACE, ENTER, or controller buttons), because that involves restructuring some of the Flutter controls to trigger animations differently, and so will be done in another PR (#41220)
This re-lands #41417 with a slight change that will hopefully not tickle the analyzer as it did before. The last time I tried to land this, the analyzer succeeded for the analyze step in Cirrus, and locally, but failed in an integration test.
This implements the keyboard shortcut handling and action invocation in order to provide a place in the infrastructure for keyboard events to trigger actions. This will allow binding of key events to actions like "move the focus to the next widget" and "activate button".
