More flexible logging from PluginInstance

I had a version of this where PluginCoreLib took a dependency on
SystemUILogLib, but that proved too difficult to get working. This
approach ended up being much simpler while accomplishing the same
objectives.

Flag: NONE
Bug: 279977555
Test: Existing tests
Change-Id: I74ee7fc7d11ac1326f3bf4ec1bb886c80d8e2d82