Cleanup IME window visibility flag logic

Cache last reported imeWindowVisibility value in IME to avoid sending
duplicate IPC calls
Remove updates to imeWindowVisibility that were not needed
Handle imeWindowVisibility updates when a service disconnects (e.g. when
the currently shown IME is being updated/uninstalled)

Test: observe navigation bar behaviour when showing/hiding IME in
  different apps, when rotating the screen with IME shown,
  when in split screen, when switching between IMEs, and when
  updating/uninstalling the IME that is currently visible
Bug: 271816203
Change-Id: I7980a68bca22009c364a635e557214da2ccf7fa9