SystemUI: Fix NPE when DozeService is destroied

When DozeService's host is not null, it can be created successful
and DozeService is added into PluginManagerImpl instance as a
PluginListener. When DozeService is stoped, it will be removed
from PluginManagerImpl instance.

When DozeService's host is null, PluginManagerImpl instance is
also null, it will cause NPE if trying to remove PluginListener
from PluginManagerImpl instance.
In this case, add null check to avoid NPE

Change-Id: I96c767d9e36c84926d5fb7f9cf2dc1a1c0ce2e08
CRs-Fixed: 2348040