on_device_model: Add multiple sessions support

Creating new session, Loading new adaptation, executing the model and
add new context should not interrupt the model execution, and we should
also support the context switch between multiple sessions.

BUG=b:331513057
TEST=Create multiple sessions, and see what will happen

Change-Id: I5cc15d990ab4c6c5275d88b155758f28d091cf1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5400817


Reviewed-by: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Yi Chou <yich@google.com>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282197}