Fix issue with Shell listeners being unbound if SysUI is restarted

- If SysUI is restarted (ie. during dev or a crash), the components
  registering the shell listeners may not be aware of this, and
  listener callbacks will not be re-registered.  Currently all the
  listeners are managed correctly (bound & later unbound) so we can
  just keep the active listener references to re-register if this
  happens.

Bug: 207142749
Test: Kill SysUI, start some apps and ensure Launcher still shows them
      in recents
Change-Id: Iba3b11ee972caf8f0b482942d38c3a7359211180