1/ Add mechanism to expose shell feature directly (for Pip)

- Also split impl and controller for ShellInit/ShellCommandHandler
  (as with ag/13502602)
- Example implementation of exposing a subset of the Pip interface to
  Launcher directly.  This has the benefit of reducing unnecessary code
  in SysUI just to pipe calls to the Shell.

  The controller implements the binder interface which is collected
  and exposed to Launcher when it binds to the overview  service
  and Launcher can call through the binders directly.

  Note: this requires the shared lib to also build with the Shell
  interfaces so changes to the Shell aidls will still require updating
  the shared lib (until the shared lib prebuilt can removed).

Bug: 180074017
Test: atest WMShellUnitTests
Test: Verify Pip calls from Launcher work

Change-Id: If048d2cd9a6b8e5014ba30c0deaed7a3e177605d
Signed-off-by: Winson Chung <winsonc@google.com>