Refactor code to PluginEnabler

SystemUI uses PackageManager to enable/disable plugin components.
Launcher cannot do this, so we abstract this logic into an interface;
Launcher will have a different implementation.

Test: "atest com.android.systemui.shared.plugins"
  build succeeds and all tests pass

Bug: 115877296
Change-Id: Ie6f11572026dafa369c99722d63d4daa0ea44fb5