Use main executor for handling key gesture events in desktop mode.

This is to fix a bug where the toast doesn't show when snap resizing a non-resizable app using keyboard shortcuts.

Test: manual test and atest DesktopModeKeyGestureHandlerTest
Fixes: 380036936
Flag: com.android.window.flags.enable_task_resizing_keyboard_shortcuts
Change-Id: Ia64a119bdf0c8aafd43b13d41a040de271be19c2