Add plugin support

- Add libs/plugin_core.jar
- Include plugin_core in Launcher3 build (it is already present other
  builds as part of the updated shared lib)
- Add PluginEnablerImpl that uses SharedPrefs to enable/disable plugin
  components
- Add src_plugins, where plugin interfaces will live. It has a build
  rule to create a jar that plugin projects will depend on.
- Copy PluginPreferencesFragment from sysui but using our implementation
  for PluginEnabler

Bug: 115877296
Change-Id: I3db54677eaceb10f92018c0f9d18920ad9ffac39