Fix tap on navbar handle when on desktop

Tapping on navbar handle was hiding the stashed taskbar handle.
When tappin on navbar handle, it starts the recents animation. Which
gets cancelled. Normally, the fullscreen app being visible, it means
launcher activity will be paused after. But when on desktop, launcher is
visible in the background and we need to manually set it to paused
state.
When recents gesture is cancelled after navbar handle tap, the gesture
end state is null. Detect this in DesktopVisibiltyController and mark
launcher as paused in this case.

Bug: 286140120
Flag: persist.wm.debug.desktop_mode_2
Test: open an app on desktop, tap on navbar, observe that user remains
      on desktop
Change-Id: Iee915026265721d42a0b722d6b1595521f20a59a