Less cereal, more stable

Sometimes when the PluginManagerImpl got around to executing the
message it posts in the constructor, the next test would be initializing
the Dependencies and it would cause a crash. Clean up this whole
thing by making the looper injectable and the message runs at
a deterministic time.

Test: runtest systemui
Change-Id: I485d72aea22aa8f6c759012d54701af8a503681b