Add plugin controls to tuner

Allow plugins to be manually turned off from within the tuner. This
screen only shows itself if at some point in time a plugin has been
active on this device.  Plugins can also serface settings there by
receiving the com.android.systemui.action.PLUGIN_SETTINGS action.

Test: Manual
Change-Id: Ifb043c85e383fc072c6445ae322293a6401a6f2c