Fix multi-window assiststructure trashing

When multiple activities within the same process
try to handle requests for AssistStructure, the
singleton mLastAssistStructure tends to trash
the old structure when a second window's request
comes in.

This change passes in a sessionId so that the
cache is only cleared if the session id changes.

Bug: 28348867
Change-Id: I07efcd933db7e48aefd25a1c95493b71bbcffe4b