Adding support for PIP actions.

- Introduced generic RemoteAction to represents an action
  that can be made across processes with an icon and text
  description based on a Notification action.
- Modified PinnedStackController to ensure that it notifies
  the listeners from the source of truth, this ensures that
  SysUI is in the right state if killed and re-registers
  itself.

Test: Enable menu & minimize in SystemUI tuner.
Test: android.server.cts.ActivityManagerPinnedStackTests
Test: #testNumPipActions

Change-Id: I5b5d0cf9de3f06b5687337d59cfb91e17355bdb1
Signed-off-by: Winson Chung <winsonc@google.com>