Clean up DozeService plugin listeners.

Also adds some additional debugging:
`adb shell dumpsys activity service SystemUI` will now dump
the plugin map. This will help us discover leaks that happen
to interact with plugins, or leaks in the plugins themselves.

(Note that plugins only work on debuggable builds.)

Bug: 72138063
Test: adb shell dumpsys activity service SystemUI
Change-Id: I328aeb9a0940404760e9121ef26a0518371b8319