SurfaceControlViewHost: Restrict disclosure of input token

Currently we send the input (channel) token from the embedded
view hierarchy to the remote embedding process via SurfacePackage.
This is used to call grantEmbeddedWindowFocus. However this could also
allow the host process to invoke updateInputChannel, which is not
desirable. To fix this we rework updateInputChannel to work in terms
of a focusGrantToken, which only carries this 1 capability.
SurfacePackage is modified to not include the input token, but rather
include this focus grant token.

Bug: 215912712
Test: Existing tests pass
Change-Id: Ia9153c54e3e36e73c2b56763b96afab9e9f20d40