SurfaceControlViewHost: Expose IPC helpers

Use of SurfaceControlViewHost requires the embeder and embedee to
establish a side-channel in order to exchange the SurfacePackage
containing the embedded hierarchy. In recent commits, SurfacePackage
has grown a built in side-channel, useful for exchanging info from
the host back in to the embedee. In this CL we make two of these APIs
public. The general motivation for making them public is as follows:
	1. onDetachedFromWindow: While public API provides a manner
	to tear down a SurfaceControlViewHost, it seems that almost
	every user will have to include an IPC method for this in their
	bespoke side-channel. Since it's required for every use case
	it seems to make sense to provide some convenience.
	2. onConfigurationChanged: At the moment there would be no way
	to change the configuration of the remote
	SurfaceControlViewHosts. Client apps are instead forced to
	create their own configuration update side-channel and recreate
	the whole SurfaceControlViewHost each time.

Bug: 200284683
Test: SurfaceControlViewHostTests
Change-Id: I6f33f0953224d7ff60dc819560d54564250bbe3f