Move plugin support to shared lib

In addition to changing imports, this required some changes to use java
7, since that's what the shared lib needs to be for launcher.

See bug for more details on this project.

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

Bug: 115877296
Change-Id: Ib79ede0a61664df0ba1a194632a345d2229777fc