Allow whitelisted plugins on user builds

Plugins should only run on user builds if explicitly whitelisted.
It's also necessary to hold a signature permission:
  com.android.systemui.permission.PLUGIN

Test: atest PluginInstanceManagerTest
Test: atest PluginManagerTest
Test: manually try to install plugin on user build (whitelisted or not)
Bug: 111414690
Change-Id: If17b13f4caef677d641cba84b491b65c8135679b