1/N Move plugin loading into PluginInstanceManager.

The PluginInstanceManager now manages plugin instances. It loads
them, when instructed to do so by the PluginManager.

This is an imperfect cl - the first of a few to come. Right now,
the PluginInstanceManager is not focused on only loading its specific
instance. This will change.

Bug: 194781951
Test: manual && atest SystemUITests
Change-Id: I2006d623c9d9d2d1020097c567fc7af56a06a83b