2/ Add mechanism to expose other features directly

- Expose interfaces for splitscreen, one handed, shell transitions,
  starting window
- Make the shell code only reference the starting window controller and
  not the exported interface

Bug: 180074017
Test: atest WMShellUnitTests
Test: Verify Pip calls from Launcher work
Change-Id: I49a5a0419996754e5e154df7af1e475268035a5a