Fix issue #22328792: Fix scalability issues in AssistStructure

We can now stream the AssistStructure across processes, avoiding
IPC size limitations for large structures.  There is also a new
API that gets called on the VoiceInteractionSession if there is
a failure retrieving the assist data.

Also fix issue #22351981: Runtime restart due to ANR in system server,
getting rid of a deadlock.

And also tweak object lifecycles to try to avoid keeping around
in an app the previous AssistStructure after we request a new one.

Change-Id: Ifb136a0d31a14e56a8db6b90768d9fc65557a17f