Give SliceManagerService a concept of pinned slices.

Allow launcher/assistant to pin slices. Also allow other apps
with access to a slice to pin it dynamically by adding a listener
to it. Dynamic pinnings can expire when the host app dies because
the binder will no longer be alive.

Public docs and unhiding will come in a later CL with CTS tests.

Bug: 68378571
Test: runtest --path frameworks/base/services/tests/uiservicestests
Change-Id: I1dd9a839f547fc949922b766ba4b54e6d50f3125